Online PHP editor | refs for MEpHD

<?php $array = [ ["444", "0081"], ["449", "0081"], ["451", "0081"], ["455", "2100"], ["469", "2100"] ]; var_export( array_values( array_reduce( $array, function($result, $row) { if (!isset($result[$row[1]])) { $result[$row[1]] = $row; // first occurrence of group, save whole row } else { $result[$row[1]][0] .= ',' . $row[0]; // not first occurrence, concat first element in group } return $result; } ) ) );