Online PHP editor | output for nfEjR

<?php $array = [ ['s1','s2','s5','s1','s25','s1','s6','s6','s1'], 2 => ['a2','a1','a4'], [], [] ]; $result = []; foreach ($array as $subarray) { if ($subarray) { $result[] = implode(',', $subarray); } } echo implode(',', $result); echo "\n\n---\n\n"; array_walk_recursive( $array, function($v) { static $first; echo $first . $v; $first = ','; } ); echo "\n\n---\n\n"; echo implode( ',', preg_match_all( '/[a-z]\d+/', json_encode($array), $out ) ? $out[0] : [] ); echo "\n\n---\n\n"; echo implode( ',', preg_split( '/\W+/', json_encode(array_values($array)), 0, PREG_SPLIT_NO_EMPTY ) );