Online PHP editor | output for AmJpY

<?php $data = [ [ 'table' => 'test', 'parent_table' => NULL, ], [ 'table' => 'test', 'parent_table' => NULL, ], [ 'table' => 'test2', 'parent_table' => 'test', ], [ 'table' => 'test4', 'parent_table' => NULL, ], [ 'table' => 'test5', 'parent_table' => 'test3', ], [ 'table' => 'test6', 'parent_table' => 'test5', ], [ 'table' => 'test3', 'parent_table' => 'test', ], ]; function reorderHierarchy($data){ $hierarchy = []; $top_level_parents = []; foreach($data as $each_data){ $hierarchy[$each_data['table']] = array(); if(is_null($each_data['parent_table'])){ if(!isset($top_level_parents[$each_data['table']])){ $top_level_parents[$each_data['table']] = 0; } $top_level_parents[$each_data['table']]++; } } foreach($data as $each_data){ if(!is_null($each_data['parent_table'])){ $hierarchy[$each_data['parent_table']][] = $each_data['table']; } } $result = []; traverseHierarchy($hierarchy,$top_level_parents,$result); return $result; } function traverseHierarchy($hierarchy,$top_level_parents,&$result){ foreach($top_level_parents as $each_parent => $occurrences){ while($occurrences-- > 0){ $result[] = [ 'table' => $each_parent, 'parent_table' => NULL ]; } traverseChildren($hierarchy,$each_parent,$result); } } function traverseChildren($hierarchy,$parent,&$result){ foreach($hierarchy[$parent] as $each_child){ $result[] = [ 'table' => $each_child, 'parent_table' => $parent ]; traverseChildren($hierarchy,$each_child,$result); } } foreach(reorderHierarchy($data) as $each_data){ echo $each_data['table']," , ",(is_null($each_data['parent_table']) ? "NULL" : $each_data['parent_table']),PHP_EOL; }