The array_filter() function is a PHP built-in function introduced in PHP 4. This PHP function is handy for filtering the array elements using a user-defined callback function.

By iterating over the array values, the array_filter() function passes them to the callback function. The callback function is user-defined. If the callback function returns true, then the current value of the array is returned to the result array.

array_filter() function Syntax:

array array_filter($array, $callback_function, $flag)

The array_filter() function takes three parameters

$array: This refers to the input array, and it is required.

$callback_function(Optional): This parameter refers to the user-defined function. This callback function does all the logic to filter the input array. If no function is given, all the array entries are equal to false.

$flag(optional): Two filters can be used to perform the filter. ARRAY_FILTER_USE_KEY  – which only passes the input array’s key to the callback function. ARRAY_FILTER_USE_BOTH – this flag indicates that the callback function will have both the key and value.

Example #1: Find all the even numbers from a given array using the array_filter() function:

<?php
# Old School way
$numbers = [2, 3, 4, 89, 101, 5,200];
function evenn($numbers)
{
    if ($numbers % 2 === 0)
        return true;
}
print_r(array_filter($numbers, 'evenn'));

# The best approach with the arrow function
# only two line of code
$even_numbers = array_filter($numbers, fn($num) => $num % 2 == 0);
print_r($even_numbers);

Example #2: Find all the people aged over 60 from an associative array

<?php
/**
 * Assume we have an array of people and 
 * want to filter out everyone who is older than 60 years old.
 */

$people = [
    [
        'name' => 'Anna',
        'age' => 78
    ],
    [
        'name' => 'Maxi',
        'age' => 40
    ],
    [
        'name' => 'Johna',
        'age' => 65
    ],
    [
        'name' => 'Donald',
        'age' => 59
    ],
    [
        'name' => 'Monir',
        'age' => 37
    ]
];

$people_over_60 = array_filter($people, fn($person)=> $person['age'] > 60, ARRAY_FILTER_USE_BOTH );
print_r($people_over_60);

#Output
Array
(
    [0] => Array
        (
            [name] => Anna
            [age] => 78
        )

    [2] => Array
        (
            [name] => Johna
            [age] => 65
        )

)

Example #3: Find all the names whose names end is ‘y’

<?php
$people = ["Johnny", "Timmy", "Bobby", "Sam", "Tammy", "Danny", "Joe"];

$names_end_with_y = array_filter($people, fn($person) => substr($person, -1 ) === 'y' );

print_r($names_end_with_y);

#Output
Array
(
    [0] => Johnny
    [1] => Timmy
    [2] => Bobby
    [4] => Tammy
    [5] => Danny
)