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'];
$filtered = array_filter(
    function ($key) use ($allowed) {
        return in_array($key, $allowed);

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'];
$filtered = array_filter(
    function ($val, $key) use ($allowed) { // N.b. $val, $key not $key, $val
        return isset($allowed[$key]) && (
            $allowed[$key] === true || $allowed[$key] === $val
); // ['foo' => 1, 'bar' => 'baz']

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