PHP infinite classification recursive function implementation

/**

*

* @param All arrays $array

* @param Current user ID $id

* @param Storage variables $str

* @return string

*/

function findIds($array,$id,$str='') {

$result = findChild($array,$id);//Get all the same items under the current node Level child node

foreach ($result as $k => $v){

//Assign value to variable

$str.=$v['id'].',';

//Call again This function displays the sibling child nodes under the child node

findIds($array,$v['id'],&$str);

}

return $str; //Return variable

}

//Get all sibling child nodes under the current node

function findChild(&$arr,$id){

$childs=array();

foreach ($arr as $k => $v) {

if($v['pid']== $id){

$childs[]=$v;

}

}

return $childs;

}