Using array_intersect with an unknown number of arrays

Hi, I have a result set coming from a database that can be of the form below $lines = array(0 => array('idA' => 1, 'idU' => 1), 1 => array('idA' => 1, 'idU' => 2), 2 => array('idA' => 2, 'idU' => 1), 3 => array('idA' => 2, 'idU' => 2), 4 => array('idA' => 3, 'idU' => 1), 5 => array('idA' => 3, 'idU' => 2)); I used the code to generate arrays based on the idA, i.e, one array containing all the idU from each idA foreach ($lines as $r) { if(!isset($list[$r['idA']])) { $list[$r['idA']] = array(); } array_push($list[$r['idA']], $r['idU']); } Now need just the idU that occurs in all. In my example I could simply $x = array_intersect($list[1], $list[2], $list[3]); This works but the problem is that I do not know in advance how many (and which) indexes I will get from that foreach since the results will differ from the database. Is there a way around this ?