Using PHP's array_filter to Filter Array Elements with Callback Functions

The array_filter function iterates over each value of an input array and passes it to a user‑defined callback. If the callback returns TRUE, the element is kept in the resulting array while preserving the original keys.

Signature :

array_filter(array $array, callable $callback = null, int $flag = 0)

Parameters array – the array to be filtered. callback – the function that determines whether an element should be kept. If omitted, all entries that are equal to FALSE are removed. flag – optional flag that changes the arguments passed to the callback. Use ARRAY_FILTER_USE_KEY to receive only the key, or ARRAY_FILTER_USE_BOTH to receive both key and value.

Return value : the filtered array.

Example 1 – Filtering odd and even numbers

<?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 :
";
print_r(array_filter($array1, "odd"));

echo "Even:
";
print_r(array_filter($array2, "even"));
?>

Output:

Odd :
Array ( [a] => 1 [c] => 3 [e] => 5 )
Even:
Array ( [0] => 6 [2] => 8 [4] => 10 )

Example 2 – Filtering truthy values (no callback)

<?php
$entry = array(0=>'foo', 1=>false, 2=>-1, 3=>null, 4=>'');
print_r(array_filter($entry));
?>

Output:

Array ( [0] => foo [2] => -1 )