php: Check if an array is associative or plain

is_assoc_array.php

<? php

function is_assoc_array($a) { 
    return (is_array($a) 
            && (array_keys($a) !== range(0, count($a) - 1)));
}

function is_plain_array($a) { 
    return  is_array($a) && !is_assoc_array($a); 
}

This works fine but it's obviously not very efficient. If you know a better way to do it please leave a reference to it.