PHP: How to use array_filter() to filter array keys? PHP: How to use array_filter() to filter array keys? arrays arrays

PHP: How to use array_filter() to filter array keys?


With array_intersect_key and array_flip:

var_dump(array_intersect_key($my_array, array_flip($allowed)));array(1) {  ["foo"]=>  int(1)}


PHP 5.6 introduced a third parameter to array_filter(), flag, that you can set to ARRAY_FILTER_USE_KEY to filter by key instead of value:

$my_array = ['foo' => 1, 'hello' => 'world'];$allowed  = ['foo', 'bar'];$filtered = array_filter(    $my_array,    function ($key) use ($allowed) {        return in_array($key, $allowed);    },    ARRAY_FILTER_USE_KEY);

Since PHP 7.4 introduced arrow functions we can make this more succinct:

$my_array = ['foo' => 1, 'hello' => 'world'];$allowed  = ['foo', 'bar'];$filtered = array_filter(    $my_array,    fn ($key) => in_array($key, $allowed),    ARRAY_FILTER_USE_KEY);

Clearly this isn't as elegant as array_intersect_key($my_array, array_flip($allowed)), but it does offer the additional flexibility of performing an arbitrary test against the key, e.g. $allowed could contain regex patterns instead of plain strings.

You can also use ARRAY_FILTER_USE_BOTH to have both the value and the key passed to your filter function. Here's a contrived example based upon the first, but note that I'd not recommend encoding filtering rules using $allowed this way:

$my_array = ['foo' => 1, 'bar' => 'baz', 'hello' => 'wld'];$allowed  = ['foo' => true, 'bar' => true, 'hello' => 'world'];$filtered = array_filter(    $my_array,    // N.b. it's ($val, $key) not ($key, $val):    fn ($val, $key) => isset($allowed[$key]) && (        $allowed[$key] === true || $allowed[$key] === $val    ),    ARRAY_FILTER_USE_BOTH); // ['foo' => 1, 'bar' => 'baz']


I needed to do same, but with a more complex array_filter on the keys.

Here's how I did it, using a similar method.

// Filter out array elements with keys shorter than 4 characters$a = array(  0      => "val 0",   "one"  => "val one",   "two"  => "val two",   "three"=> "val three",   "four" => "val four",   "five" => "val five",   "6"    => "val 6"); $f = array_filter(array_keys($a), function ($k){ return strlen($k)>=4; }); $b = array_intersect_key($a, array_flip($f));print_r($b);

This outputs the result:

Array(    [three] => val three    [four] => val four    [five] => val five)