(PHP 4 >= 4.0.6, PHP 5, PHP 7)
array_filter — Filters elements of an array using a callback function
   Iterates over each value in the array
   passing them to the callback function.
   If the callback function returns true, the
   current value from array is returned into
   the result array. Array keys are preserved.
  
arrayThe array to iterate over
callbackThe callback function to use
       If no callback is supplied, all entries of
       array equal to FALSE (see
       converting to
        boolean) will be removed.
      
flag
       Flag determining what arguments are sent to callback:
       
ARRAY_FILTER_USE_KEY - pass key as the only argument
         to callback instead of the value
        ARRAY_FILTER_USE_BOTH - pass both value and key as
         arguments to callback instead of the value
        Returns the filtered array.
| Version | Description | 
|---|---|
| 5.6.0 | Added optional flagparameter and constantsARRAY_FILTER_USE_KEYandARRAY_FILTER_USE_BOTH | 
Example #1 array_filter() example
<?php
function odd($var)
{
    // returns whether the input integer is odd
    return($var & 1);
}
function even($var)
{
    // returns whether the input integer is even
    return(!($var & 1));
}
$array1 = array("a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5);
$array2 = array(6, 7, 8, 9, 10, 11, 12);
echo "Odd :\n";
print_r(array_filter($array1, "odd"));
echo "Even:\n";
print_r(array_filter($array2, "even"));
?>
The above example will output:
Odd :
Array
(
    [a] => 1
    [c] => 3
    [e] => 5
)
Even:
Array
(
    [0] => 6
    [2] => 8
    [4] => 10
    [6] => 12
)
Example #2 array_filter() without
    callback
<?php
$entry = array(
             0 => 'foo',
             1 => false,
             2 => -1,
             3 => null,
             4 => ''
          );
print_r(array_filter($entry));
?>
The above example will output:
Array
(
    [0] => foo
    [2] => -1
)
Example #3 array_filter() with 
    flag
<?php
$arr = ['a' => 1, 'b' => 2, 'c' => 3, 'd' => 4];
var_dump(array_filter($arr, function($k) {
    return $k == 'b';
}, ARRAY_FILTER_USE_KEY));
var_dump(array_filter($arr, function($v, $k) {
    return $k == 'b' || $v == 4;
}, ARRAY_FILTER_USE_BOTH));
?>
The above example will output:
array(1) {
  ["b"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(2)
  ["d"]=>
  int(4)
}
If the array is changed from the callback function (e.g. element added, deleted or unset) the behavior of this function is undefined.