PHP - Simplexml_file_load - Child May Not Exist

i have error where my code should update existing data where id exist, it get updated ..but the others data is disappeared .only the data updated is remained 

$sql1 ="select*from semakan_dokumen where email='$stdEmail'";
$sqlsearch1 = mysqli_query($dbconfig,$sql1);
$resultcount1 = mysqli_num_rows($sqlsearch1);

if($resultcount1 > 0){
$query1=("UPDATE  semakan_dokumen set student_id='$noMatrik', email= '$stdEmail', surat_tawaran='$fileName', ic='$fileName1',sijil_lahir='$fileName2',sijil_spm= '$fileName3',sijil_sekolah= '$fileName4', sijil_dip= '$fileName5',sej_julai='$fileName6',bm_julai='$fileName7',muet='$fileName8', mid1='$fileName9',yuran= '$fileName10',umpa_bend1= '$fileName11',umpa_bend2='$fileName12',bpkp='$fileName13', penaja='$fileName14',kesihatan= '$fileName15', jhepa='$fileName16' where email= '$stdEmail' ");

}else{


//filezip
$query1 = "INSERT INTO semakan_dokumen (email,surat_tawaran,ic,sijil_lahir,sijil_spm,sijil_sekolah,sijil_dip,sej_julai,bm_julai,muet,mid1,yuran,umpa_bend1,umpa_bend2,bpkp,penaja,kesihatan,jhepa) VALUES ('$stdEmail','$fileName','$fileName1','$fileName2','$fileName3','$fileName4','$fileName5','$fileName6','$fileName7','$fileName8','$fileName9','$fileName10','$fileName11','$fileName12','$fileName13','$fileName14','$fileName15','$fileName16')";
}

Hi I am trying to create a hierarchy array from our mysql database.

What i have so far is:

//instantiate DB & connect
$database = new Database();
$db = $database->connect();

//instantiate question object

$question = new Question($db);

//query
    $results = $question->read(); 
   
      //check for data
    if ($results) {

     $question_arr['questions'] = array();
     $child_arr = array();
     
    while ($row = $results->fetch(PDO::FETCH_ASSOC)) {
        
        array_push($question_arr['questions'], $row);

        $question_arr['questions'][] = buildTree($row);

    }
        print_r($question_arr['questions']);
} else {
    echo json_encode(array('message'=>'nothing here!.'));
}


function buildTree($child) {
    $branch = array();
   foreach ($child as $row) {
    
    $childresults = $question->read_children($row['parentid']); 
    
           while ($childrow =$childresults->fetch(PDO::FETCH_ASSOC)) {
             
            $children = buildTree($childrow);

            if ($children) {
             $row['children'] = $children;
            }
         
            $branch[] = $childrow;
        }
    }
   //}
    return $branch;
}

in my Questions.php file:

<?php
class Question{
    private $conn;
    public $id;
    public $childid;
    public $parentid;
    public $question;
    public $pageid;
    public $numclicks;
    public $typeid;
    public $date_created;

 
     // Constructor with DB
     public function __construct($db) {
        $this->conn = $db;
      }

   public function read() {
        //create query(sql statement)
      $query = 'SELECT id as parentid, question, pageid, typeid,numclicks,date_created from ct_questions';
      // Prepare statement
      $stmt =$this->conn->prepare($query);
      // Execute query
      $stmt->execute();
      return $stmt;
    }

public function read_children($parentid)
{
//This is from a View I created in Mysql
    $query = 'SELECT parent_question, parentid, parent_pageid, child_question, childid, child_pageid from ct_vquestion_parent_child_lookup where parentid='.$parentid.'';
    // Prepare statement
    $stmt = $this->conn->prepare($query);
  
    // Execute query
    $stmt->execute();
   
    return $stmt;
}
}

the current errors i am getting a

<br /> <b>Notice</b>: Undefined variable: question in <b>C:\htdocs\api\questions\read.php</b> on line <b>41</b><br /> <br /> <b>Fatal error</b>: Uncaught Error: Call to a member function read_children() on null in C:\htdocs\api\questions\read.php:41 Stack trace: #0 C:\htdocs\api\questions\read.php(28): buildTree(Array) #1 {main} thrown in <b>C:\htdocs\api\questions\read.php</b> on line <b>41</b><br />