<?php $treeData = [ [11, 12, 13, 14], [ 21, 22, [ 231, 232, 234, 235, [ 2361, 2362, 2363 ] ] ], [31, 32, [ 331, 332, 333 ] ] ]; function depth($tree) { $maxDepth = 0; $depth = -1; $tree = str_split(preg_replace('/[^\{\}]+/', '', serialize($tree))); foreach ($tree as $char) { $char == '{' ? $depth++ : $depth--; $maxDepth = max($maxDepth, $depth); } return $maxDepth; } echo depth($treeData);