$only_integers === array_filter($only_integers, 'is_int'); // true
$letters_and_numbers === array_filter($letters_and_numbers, 'is_int'); // false
It would help you in the future to define two helper, higher-order functions:
/**
* Tell whether all members of $array validate the $predicate.
*
* all(array(1, 2, 3), 'is_int'); -> true
* all(array(1, 2, 'a'), 'is_int'); -> false
*/
function all($array, $predicate) {
return array_filter($array, $predicate) === $array;
}
/**
* Tell whether any member of $array validates the $predicate.
*
* any(array(1, 'a', 'b'), 'is_int'); -> true
* any(array('a', 'b', 'c'), 'is_int'); -> false
*/
function any($array, $predicate) {
return array_filter($array, $predicate) !== array();
}