Question

I know about array_intersect_key which returns duplicate keys of the first parameter in any of the following parameters.

However I was wondering which would be the easiest way to find duplicate keys across more than two arrays at once? Does PHP offer such a function or do I need to do it with multiple calls?

Given

$a1 = array('hello'=>1, 'tag'=>1);
$a2 = array('moin'=>1);
$a3 = array('moin'=>1, 'tag'=>1);

Duplicate keys across all three arrays are moin and tag.

I thought so far about calling array_intersect_keys on each possible pair of parameters (in a function accepting a 2-n number of arrays as parameters) but have problems to actually find all possible combinations. And perhaps there is a much more easier way to do this.

Answer 1

I think your idea is good – call it on every possible combination of arrays. Two nested for loops should be enough to get all combinations:

function array_duplicate_keys() {
    $arrays = func_get_args();
    $count = count($arrays);
    $dupes = array();
    // Stick all your arrays in $arrays first, then:
    for ($i = 0; $i < $count; $i++) {
        for ($j = $i+1; $j < $count; $j++) {
            $dupes += array_intersect_key($arrays[$i], $arrays[$j]);
        }
    }
    return array_keys($dupes);
}