If you want a quick way to remove NULL, FALSE and Empty Strings (""), but leave values of 0 (zero), you can use the standard php function strlen as the callback function:
eg:
<?php

// removes all NULL, FALSE and Empty Strings but leaves 0 (zero) values
$result = array_filter( $array, 'strlen' );

?>

If you want to use array_filter with a class method as the callback, you can use a psuedo type callback like this:

<?php
class Test
{
    public function doFilter($array)
    {
        return array_filter($array, array($this, 'callbackMethodName'));
    }

    protected function callbackMethodName($element)
    {
        return $element % 2 === 0;
    }
}

$example = new Test;
print_r($example->doFilter(range(1, 10)));
?>

Will return even numbers.

In case you are interested (like me) in filtering out elements with certain key-names, array_filter won't help you. Instead you can use the following:

<?php
$arr = array( 'element1' => 1, 'element2' => 2, 'element3' => 3, 'element4' => 4 );
$filterOutKeys = array( 'element1', 'element4' );

$filteredArr = array_diff_key( $arr, array_flip( $filterOutKeys ) )
?>

Result will be something like this:
['element2'] => 2
['element3'] => 3

Some of PHP's array functions play a prominent role in so called functional programming languages, where they show up under a slightly different name:

<?php
  array_filter() -> filter(),
  array_map() -> map(),
  array_reduce() -> foldl() ("fold left")
?>

Functional programming is a paradigm which centers around the side-effect free evaluation of functions. A program execution is a call of a function, which in turn might be defined by many other functions. One idea is to use functions to create special purpose functions from other functions.

The array functions mentioned above allow you compose new functions on arrays.

E.g. array_sum = array_map("sum", $arr).

This leads to a style of programming that looks much like algebra, e.g. the Bird/Meertens formalism.

E.g. a mathematician might state

  map(f o g) = map(f) o map(g)

the so called "loop fusion" law.

Many functions on arrays can be created by the use of the foldr() function (which works like foldl, but eating up array elements from the right).

I can't get into detail here, I just wanted to provide a hint about where this stuff also shows up and the theory behind it.

Here's a function that will filter a multi-demensional array. This filter will return only those items that match the $value given

<?php
    /*
     * filtering an array
     */
    function filter_by_value ($array, $index, $value){
        if(is_array($array) && count($array)>0) 
        {
            foreach(array_keys($array) as $key){
                $temp[$key] = $array[$key][$index];
                
                if ($temp[$key] == $value){
                    $newarray[$key] = $array[$key];
                }
            }
          }
      return $newarray;
    }
?>

Example:

<?php
$results = array(
   0 => array('key1' => '1', 'key2' => 2, 'key3' => 3),
   1 => array('key1' => '12', 'key2' => 22, 'key3' => 32)
);

$nResults = filter_by_value($results, 'key2', '2');
?>

Output :

array(
    0 => array('key1' => '1', 'key2' => 2, 'key3' => 3)
);

You can access the current key of array by passing a reference to array into callback function and call key() and next() method in the callback function:
<?php
$data = array('first' => 1, 'second' => 2, 'third' => 3);
$data = array_filter($data, function ($item) use (&$data) {
    echo "Filtering key ", key($data), '<br>', PHP_EOL;
    next($data);
    return false;
});
?>

However be careful with array internal pointer or use reset() method before calling array_filter().

This function filters an array and remove all null values recursively.

<?php
  function array_filter_recursive($input)
  {
    foreach ($input as &$value)
    {
      if (is_array($value))
      {
        $value = array_filter_recursive($value);
      }
    }
   
    return array_filter($input);
  }
?>

Or with callback parameter (not tested) :

<?php
  function array_filter_recursive($input, $callback = null)
  {
    foreach ($input as &$value)
    {
      if (is_array($value))
      {
        $value = array_filter_recursive($value, $callback);
      }
    }
   
    return array_filter($input, $callback);
  }
?>

be careful with the above function "array_delete"'s use of the stristr function, it could be slightly misleading. consider the following:

<?php
function array_delete($array, $filterforsubstring){
    $thisarray = array ();
    foreach($array as $value)
        if(stristr($value, $filterforsubstring)===false && strlen($value)>0)
            $thisarray[] = $value;
    return $thisarray;
}

function array_delete2($array, $filterforstring, $removeblanksflag=0){
    $thisarray = array ();
    foreach($array as $value)
        if(!(stristr($value, $filterforstring) && strlen($value)==strlen($filterforstring))
                && !(strlen($value)==0 && $removeblanksflag))
            $thisarray[] = $value;
    return $thisarray;
}

function array_delete3($array, $filterfor, $substringflag=0, $removeblanksflag=0){
    $thisarray = array ();
    foreach($array as $value)
        if(
            !(stristr($value, $filterfor)
                && ($substringflag || strlen($value)==strlen($filterfor))
            )
            && !(strlen($value)==0 && $removeblanksflag)
        )
            $thisarray[] = $value;
    return $thisarray;
}

$array1 = array ('the OtHeR thang','this', 'that', 'OtHer','', 9, 101, 'fifty', ' oTher', 'otHer ','','other','Other','','other blank things');

echo "<pre>array :\n";
print_r($array1);

$array2=array_delete($array1, "Other");

echo "array_delete(\$array1, \"Other\"):\n";
print_r($array2);

$array2=array_delete2($array1, "Other");

echo "array_delete2(\$array1, \"Other\"):\n";
print_r($array2);

$array2=array_delete2($array1, "Other",1);

echo "array_delete2(\$array1, \"Other\",1):\n";
print_r($array2);

$array2=array_delete3($array1, "Other",1);

echo "array_delete3(\$array1, \"Other\",1):\n";
print_r($array2);

$array2=array_delete3($array1, "Other",0,1);

echo "array_delete3(\$array1, \"Other\",0,1):\n";
print_r($array2);
?>

If you want to pass the key to the callback function before PHP 5.6.0 (when the flag parameter wasn't implemented):

<?php

$result = array_filter(array_keys($array), 'is_int');

?>

Here is how you could easily delete a specific value from an array with array_filter:

<?php
$array = array (1, 3, 3, 5, 6);
$my_value = 3;
$filtered_array = array_filter($array, function ($element) use ($my_value) { return ($element != $my_value); } );
print_r($filtered_array);
?>

output:

Array
(
    [0] => 1
    [3] => 5
    [4] => 6
)

You can use array_filter from within a class to access a protected method from that same class:

<?php

class Bar {
    public function foo()
    {
        $array1 = array("a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5);

        print_r(array_filter($array1, array($this, 'naz')));
    }

    protected function baz($var)
    {
        return($var & 1);
    }
}

$bar = new Bar();
$bar->foo();
?>

To get rid of all white space in an array without looping.
<?php
   $array = array(5, "   ", 2, NULL, 13, "", 7, "\n", 4, "\t");
   print_r($array);
   $result = array_filter($array, create_function('$a','return preg_match("#\S#", $a);'));                
   print_r($result);
?>
Array
(
    [0] => 5
    [1] =>   
    [2] => 2
    [3] =>
    [4] => 13
    [5] =>
    [6] => 7
    [7] =>

    [8] => 4
    [9] =>    
)
Array
(
    [0] => 5
    [2] => 2
    [4] => 13
    [6] => 7
    [8] => 4
)

If you have an array of KV pairs and you want all the items where a value is X, you dont need to make a callback for array_filter. You can use array_intersect:

<?php
print_r(array_intersect(
  array(
    'a' => 1,
    'b' => 1,
    'c' => 1,
    'd' => 2,
    'e' => 2,
    'f' => 2,
  ),
  array(1)
));

Array
(
    [a] => 1
    [b] => 1
    [c] => 1
)
?>

The advantage of this approach is you can pass variables into the second array without needing to worry about variable scope and function parameters for array_filter.

My favourite use of this function is converting a string to an array, trimming each line and removing empty lines:

<?php
$array = array_filter(array_map('trim', explode("\n", $string)), 'strlen');
?>

Although it states clearly that array keys are preserved, it's important to note this includes numerically indexed arrays. You can't use a for loop on $array above without processing it through array_values() first.

I use the following to see if a array consist of scalar values or null, but of course you could mix it up using any of the is_ functions.

<?php
if(count($data) !== count(array_filter($data, 'is_scalar') + array_filter($data, 'is_null'))) {
  throw new Exception('Array did not consist of scalar and null values');
}
?>

I needed to get the complete array element of a multilevel array where the values of some elements where 'x' or 'y'. Kind of a WHERE  AND in a MySQL Query..

SO if you need to filter an array where 'X' = Y and 'a' = b, here you go.

<?php
/*
* getArrayElement() by @MmO12
*
* SELECT an element of  an $array...
* *$array = array( 0 => array('index1'  => 'a', 'index2' => 'b', 'value' => 'hello'),
                1 => array('index1'  => 'a', 'index2' => 'c', 'value' => 'bye'));
*
* WHERE its $indexs values  are equal to...
* * $indexs = array('index1' => 'a', 'index2'=>'b');
*/
function getArrayElement($array, $indexs, $justvalsplease = false){
    $newarray = false;
    //verificamos el array
    if(is_array($array) && count($array)>0){
       
        //verify indexs and get # of indexs
        if(is_array($indexs) && count($indexs)>0) $ninds = count($indexs);
        else return false;
       
        //search for coincidences
        foreach(array_keys($array) as $key){

            //index value coincidence counter.
            $count = 0;
           
            //for each index we search           
            foreach($indexs as $indx => $val){
               
                //if index value is equal then counts
                if($array[$key][$indx] == $val){
                    $count++;
                }
            }
            //if indexes match, we get the array elements :)
            if($count == $ninds){
               
                //if you only need the vals of the first coincidence
                //witch was my case by the way...
                if($justvalsplease) return $array[$key];
                else $newarray[$key] = $array[$key];
            }
        }
    }
  return $newarray;
}
?>

/*******
Example
********/
<?php
   //array
    $array = array( 0 => array('index1'  => 'a', 'index2' => 'b', 'value' => 'hello'),
                  1 => array('index1'  => 'a', 'index2' => 'c', 'value' => 'bye'),
                  2 => array('index1'  => 'b', 'index2' => 'a', 'value' => 'hi'));

    //indexes
    $indexs = array('index1' => 'a', 'index2'=>'b');

    //aaand we get the element we want.
    $element = getArrayElement($array, $indexs);

        print_r($element);

    /***prints ***/
    Array
    (
        [0] => Array
            (
                [index1] => a
                [index2] => b
                [value] => hello
            )
   
    )?>

IF you want only the array inside the elements just set $justvalsplease true like this
<?php
     $element = getArrayElement($array, $indexs,true);
?>

And you will get just the element content
<?php
Array
(
    [index1] => a
    [index2] => b
    [value] => hello
)
?>

Here is key-passed array_filter function.

<?php
function arrayfilter(array $array, callable $callback = null) {
    if ($callback == null) {
        $callback = function($key, $val) {
            return (bool) $val;
        };
    }
    $return = array();
    foreach ($array as $key => $val) {
        if ($callback($key, $val)) {
            $return[$key] = $val;
        }
    }
    return $return;
}

$test_array = array('foo', 'a' => 'the a', 'b' => 'the b', 11 => 1101, '', null, false, 0);

$array = arrayfilter($test_array, function($key, $val) {
   return is_string($key);
});
print_r($array);
/*
Array
(
    [a] => the a
    [b] => the b
)
*/

$array = arrayfilter($test_array);
print_r($array);
/*
Array
(
    [0] => foo
    [a] => the a
    [b] => the b
    [11] => 1101
)
*/
?>

Read "callback" parameter note with understanding (as well as "converting to boolean" chapter). Keep in midn, that 0, both:

* integer: 0 and
* float: 0.00

evaluates to boolean FALSE! And therefore all array nodes, that have such value WILL ALSO BE FILTERED by array_filter(), with default call back. Unless you provide your own callback function, that will (for example) filter only empty strings and NULLs, but leave "zeros" untouched.

Some people (including me) might be surprised to find this out.

If you want a quick way to Find Numbers ( remove NULL, FALSE and Empty Strings (""), all Strings) but leave values of 0 (zero)
eg:
<?php
/*
Find Numbers (removes all NULL, FALSE and Empty Strings, all Strings) but leaves 0 (zero) values
*/

$result = array_filter( array( "0",0,1,2,3,'text' ) , 'is_numeric' );

var_dump($result);
/*
  array (size=5)
  0 => string '0' (length=1)
  1 => int 0
  2 => int 1
  3 => int 2
  4 => int 3
*/
?>

If you need this function to return falsy results (like "0"), yet discard truly empty ones (that have a 0 string length, like null, false, ''), you can use strlen() as the callback:

<?php
array_filter($array, 'strlen');
?>

<?php
// ARRAY FILTER RECURSIVE USING CLASS, STATIC METHOD, AND ANONYMOUS CALLBACK FUNCTION
// NOTE THAT THE CALLBACK HAS ACCESS TO BOTH THE KEY AND VALUE

// THE CLASS (FOR YOU TO COPY)
class ArrayUtil
{
    public static function FilterRecursive(Array $source, $fn)
    {
        $result = array();
        foreach ($source as $key => $value)
        {
            if (is_array($value))
            {
                $result[$key] = self::FilterRecursive($value, $fn);
                continue;
            }
            if ($fn($key, $value))
            {
                $result[$key] = $value; // KEEP
                continue;
            }
        }
        return $result;
    }
}

// EXAMPLE ANONYMOUS CALLBACK FUNCTION
$fn = function ($key, $value)
{
    if (strpos($key, 'drop') !== FALSE)
    {
        return FALSE; // DROP
    }
    return TRUE; // KEEP
};

// EXAMPLE PRE FILTER TEST DATA
$preFilter = array(
    'a' => 'one',
    'b' => array(
        'example_drop' => 'filter me out',
        'example_keep' => 'keep me',
    ),
    'c' => 'three',
);

// EXAMPLE USAGE CODE
echo '// print_r($preFilter);' . "\n";
print_r($preFilter);
$postFilter = ArrayUtil::FilterRecursive($preFilter, $fn);
echo "\n";
echo '// print_r($postFilter);' . "\n";
print_r($postFilter);

/* OUTPUT OPEN
// print_r($preFilter);
Array
(
    [a] => one
    [b] => Array
        (
            [example_drop] => filter me out
            [example_keep] => keep me
        )

    [c] => three
)

// print_r($postFilter);
Array
(
    [a] => one
    [b] => Array
        (
            [example_keep] => keep me
        )

    [c] => three
)
OUTPUT CLOSE */

If you're using filter_input_array, the values will be null on failure and anything else on success. Because array_filter by default removes false, 0 and "" you need to do extra work like this:
<?php
$input_array = filter_input_array(INPUT_GET, array(
  'var1' => FILTER_VALIDATE_BOOLEAN,
  'var2' => FILTER_VALIDATE_INT
));

array_filter($input_array, function($a) { return !is_null($a) });
?>

A function that allows filtering an array by keys:

<?php
function array_filter_key( $input, $callback ) {
    if ( !is_array( $input ) ) {
        trigger_error( 'array_filter_key() expects parameter 1 to be array, ' . gettype( $input ) . ' given', E_USER_WARNING );
        return null;
    }
   
    if ( empty( $input ) ) {
        return $input;
    }
   
    $filteredKeys = array_filter( array_keys( $input ), $callback );
    if ( empty( $filteredKeys ) ) {
        return array();
    }
   
    $input = array_intersect_key( array_flip( $filteredKeys ), $input );
   
    return $input;
}

?>

Example:

<?php
$input = array_flip( range( 'a', 'z' ) );

$consonants = array_filter_key( $arr, function( $elem ) {
    $vowels = "aeiou";
    return strpos( $vowels, strtolower( $elem ) ) === false;
} );
?>

Outputs:

array(21) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
  ["d"]=>
  int(3)
  ["f"]=>
  int(5)
  ["g"]=>
  int(6)
  ["h"]=>
  int(7)
  ["j"]=>
  int(9)
  ["k"]=>
  int(10)
  ["l"]=>
  int(11)
  ["m"]=>
  int(12)
  ["n"]=>
  int(13)
  ["p"]=>
  int(15)
  ["q"]=>
  int(16)
  ["r"]=>
  int(17)
  ["s"]=>
  int(18)
  ["t"]=>
  int(19)
  ["v"]=>
  int(21)
  ["w"]=>
  int(22)
  ["x"]=>
  int(23)
  ["y"]=>
  int(24)
  ["z"]=>
  int(25)
}

Here's an easy way to get a combination of keys and values, such that if you don't specify a key you will get the value, and if you do specify a key you will get the key:

<?php
$array = array('One' => 'First', 'Second', 'Third', 'Four' => 'Fourth', 'Fifth');

var_dump(array_keys($array));

$names = array_filter(array_keys($array), 'is_string') + array_values($array);
ksort($array);

var_dump($names);
?>

Outputs:

array(5) {
  [0]=>string(3) "One"
  [1]=>int(0)
  [2]=>int(1)
  [3]=>string(4) "Four"
  [4]=>int(2)
}
array(5) {
  [0]=>string(3) "One"
  [1]=>string(6) "Second"
  [2]=>string(5) "Third"
  [3]=>string(4) "Four"
  [4]=>string(5) "Fifth"
}

Without using ksort, the keys appear before the fallback values, instead of inline and appearing like they replace them, however the keys are intact (which is why ksort works) so it's only execution order.

Regarding comment about trimming empty strings, the code posted will get into an infinite loop if the array is reduced to zero elements. The following might be better:

<?php
function array_trim($array) {
    while (!empty($array) and strlen(reset($array)) === 0) {
        array_shift($array);
    }
    while (!empty($array) and strlen(end($array)) === 0) {
        array_pop($array);
    }
    return $array;
}
?>

This function trims empty strings from the beginning and end of an array.
It's useful when outputing plaintext files on a page and you want to skip empty lines at the beginning and end, but not within the text.

<?php
function array_trim($array) {
    while (strlen(reset($array)) === 0) {
        array_shift($array);
    }
    while (strlen(end($array)) === 0) {
        array_pop($array);
    }
    return $array;
}
?>

You might want to trim each element too.

Just a simplification of my function from last month.

<?php
function partition($input, $callback=null)
{
    if(is_null($callback))
        $true = array_filter($input);
    else
        $true = array_filter($input, $callback);

    $false = array_diff_key($input, $true);
}
?>

Here is a way to get customizable filter

<?php
function blablabla() {
    ....
    $new = getNewUidls();
    class UidlFilter {
        function UidlFilter($uidls) { $this->uidls = $uidls; }
        function filter($metamsg) { return in_array($metamsg['uidl'], $this->uidls); }
    }
    $msglist = array_filter($msglist, array(new UidlFilter($new), 'filter');
    ....
}
?>

If you have a form with multiple checkbox having ID element as value for selection in a list, you probably have a SQL request like :
$req="SELECT ... WHERE ID IN (".implode(',', array_keys($choices)).")";
without quote before and after choices keys because they are numeric values... but in fact, they can be string values, and a SQL injection problem.

with array_filter, you can easily filter bad values :
<?php
    $choices=array('A'=>'on', -1=>'on', 0=>'on', 1=>'on', 12=>'on', "1)or 1=1--"=>'on');
    print_r($choices);
    $choices=array_filter(array_keys($choices), 'is_numeric');
    print_r($choices);
?>
will print :
<?php
Array
(
    [A] => on
    [-1] => on
    [0] => on
    [1] => on
    [12] => on
    [1)or 1=1--] => on
)
Array
(
    [1] => -1
    [2] => 0
    [3] => 1
    [4] => 12
)
?>

For any type of array. Basead in redshift code.

<?php
function array_clean ($array, $todelete = false, $caseSensitive = false) {
    foreach($array as $key => $value) {
        if(is_array($value)) {
            $array[$key] = array_clean($array[$key], $todelete, $caseSensitive);
        }
        else {
            if($todelete) {
                if($caseSensitive) {
                    if(strstr($value ,$todelete) !== false)
                        unset($array[$key]);
                }
                else {
                    if(stristr($value, $todelete) !== false)
                        unset($array[$key]);
                }
            }
            elseif (empty($value)) {
                unset($array[$key]);
            }
        }
    }
    return $array;
}
?>

Here is a nice little function which will apply a callback function recursively over a multidimensional array. If the callback function returns false, then it replaces the value of the array with $filtered_ouput. This function gracefully handles objects inside of arrays (and objects within objects within arrays, etc). It is specifically designed for your callback function to process on the array key's (unlike normal array_filter which filters on the values), but it could work on the array values depending on your test criteria (YMMV).

<?PHP

function array_key_filter_multi($array, $callback, $filtered_output = "")
{
   $ret = array();
   foreach($array as $key=>$value) {
       if($callback($key,$value)) {
           if(is_array($value)) {
               $ret[$key] = array_key_filter_multi($value, $callback, $filtered_output);
           }
           elseif(is_object($value)) {
               $ret[$key] = array_key_filter_multi(get_object_vars($value), $callback, $filtered_output);
           }
           else {   
               $ret[$key]=$value;
           }
       }
       else {
           $ret[$key]=$filtered_output;
       }
   }
   return $ret;
}

?>

We use this to filter redundant data from debugging output. An example usage is:

<?

$callback_func = create_function('$key, $value', 'return ($key == "db" || $key == "smarty") ? false : true;');
echo "<PRE>" . print_r(array_key_filter_multi($_SESSION, $callback_func, "**filtered by function**"), true) . "</PRE>";

?>

Which filters all keys with "db" or "smarty" as their name (including objects which have a reference to those variables). The output of the above in a test case I did is the following:

Array
(
    [userdata] => Array
        (
            [sid] => a130e675d380e0e9fe47897922d719ac
            [not_in_db] =>
            [user_id] => 1
            [session_id] => 154
            [permissions] => 1
            [username] => tester
        )
    [systemobjects] => Array
        (
            [db] => **filtered by function**
            [smarty] => **filtered by function**
        )
)

The following function modifies the supplied array recursively so that filtering is performed on multidimentional arrays as well, while preserving keys.

<?php
function array_cleanse(&$arr){

$temp = array();
reset($arr);
if(count($arr) == 0) return "";

foreach($arr as $key=>$val):

(is_array($val))? array_cleanse($val) : NULL;
($val)? $temp[$key] = $val : NULL;

endforeach;

$arr = $temp;
reset($arr);
}
?>

$arr1 = array('a'=>20,'b'=>array(''),'c'=>array(1,0,2),'d'=>0);
array_cleanse($arr1);
$arr1 will be array('a'=>20,'c'=>array(1,2))

array_filter may not be used as it does not modify the array within itself.

I was looking for a function to delete values from an array and thought I had found it in array_filter(), however, I *didn't* want the keys to be preserved *and* I needed blank values cleaned out of the array as well. I came up with the following (with help from many of the above examples):

<?php
function array_delete($array, $filterfor){
  $thisarray = array ();
  foreach($array as $value)
    if(stristr($value, $filterfor)===false && strlen($value)>0)
      $thisarray[] = $value;
  return $thisarray;
}

$array1 = array ('OtHeR','this', 'that', 'Other','', 9, 101, 'fifty', 'other','','');

echo "<pre>array :\n";
print_r($array1);

$array2=array_delete($array1, "Other");

echo "filtered:\n";
print_r($array2);
?>

breich at reich-consulting dot net  made an awesome function, but it has a poor performance, and is bloated.

here is a shorter version:
<?php
function array_remove_keys($array, $keys = array(), $callback=null) {
    if(empty($array) || (! is_array($array))) {
        return $array;
    }

    if(is_string($keys) && !is_array($keys=explode(',',$keys)))
    {
        return $array;
    }
   
    foreach($keys as $key) {
        $key=trim($key,' ');
        if(!$callback || !$callback($array[$key],$key))
            unset($array[$key]);
    }
    return $array;
}
?>

i decided to let here stay 2 conditions, even though they can be rewritten to 1.

unsetting is faster than array_diff_keys.

sometimes, people separate strings like 'a, b, c' or 'a , b , c'...

i added support for those, since they are essentially the same list.

i also added a callback option, which you specify a function and it will give the value and the key to it.

p.s.: php uses short-circuit evaluation. example:
!$callback || !$callback($array[$key],$key)

if $callback is empty, it doesn't try to run $callback().
why?
because !$callback || !$callback($array[$key],$key) will be true, no matter the value of !$callback().
so, to save resources and time, php jumps if !$callback is true