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)