Fatal error: Maximum function nesting level of '100' reached

[jemz]

I followed this hierarchical-data-database

but the problem is that I get Fatal error: Maximum function nesting level of ‘100’ reached, I don’t know why.

here is my code

         public function display_children($parent,$level){
            $cmd = $this->connection->prepare('SELECT user_id,p_id,position from treetbl where p_id= ? order by user_id');

          $cmd->execute(array($parent));
    
          $rowdata=array();
        
          while ( $row =  $cmd->fetch(PDO::FETCH_ASSOC)) {
           
              
           
                  $rowdata[] = [['v' => $row['user_id'], 'f' => $row['user_id']], (string)$row['p_id'], $row['user_id']];
             

             $this->display_children($row['p_id'], $level + 1);
          }


          $cmd = null;
          return json_encode($rowdata);

     }

here is my table data

user_id   p_id
null          1

1             5

1             6

5             7

5              8

7             9

7            10              

Thank you in advance.

[swader]

See this for how to avoid this issue and for an explanation on why it occurs.