Filter array by its keys using an array of allowed keys

The callback function in array_filter() only passes in the array’s values, not the keys.

If I have:

$my_array = array("foo" => 1, "hello" => "world");

$allowed = array("foo", "bar");

What’s the best way to delete all keys in $my_array that are not in the $allowed array?

Desired output:

$my_array = array("foo" => 1);