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That probably requires some explanation. If you have foreach ($test as $k => $rec) { then a copy of the array is passed in $rec. Any changes to $rec are made only to that copy. However, with foreach ($test as $k => &$rec) { the address of the original array is passed (ie by reference) so any changes are made to the original array. My main sources of learning have been the manual at php.net, tips picked up in these forums and via Google and lots of practice/experimentation. I often see phpdelusions.net cited as a good source.

OK - I've added the sort usort($test, fn($a, $b) => $b['itemCount']<=>$a['itemCount']); // sort descending itemCount $seen = []; foreach ($test as $k => &$rec) { $rec['rolanID'] = array_diff($rec['rolanID'], $seen); // find new ids if ($rec['rolanID']) { // if there are some new ones ... $rec['itemCount'] = count($rec['rolanID']); // count them $seen = array_merge($seen, $rec['rolanID']); // add the new ones to those already seen } else unset($test[$k]); // if no ids, remove the array item } and I now get this (no duplicate 123)... Array ( [0] => Array ( [supplier] => TEST2 DEPO [rolanID] => Array ( [0] => 456 [1] => 188 [2] => 200 [3] => 123 ) [itemCount] => 4 ) [1] => Array ( [supplier] => TEST DEPO [rolanID] => Array ( [1] => 234 ) [itemCount] => 1 ) [2] => Array ( [supplier] => DIFFERENT DEPO [rolanID] => Array ( [0] => 897 [1] => 487 [2] => 100 ) [itemCount] => 3 ) )