Codeigniter Genrate XML Envelope

PHP-Dom-Document-CI

<?php

defined('BASEPATH') OR exit('No direct script access allowed');

class XML extends CI_Controller {
    protected $xml;
    public function __construct() {
        parent::__construct();
        $this->xml = new DOMDocument('1.0', 'utf8');
        $this->xml->formatOutput = true;
        $this->xml->preserveWhiteSpace = false;
    }
    public function index(){
        $startTag1 = ['field1'=>'field1','field2'=>'field2','field3'=>'field3'];
        $startTag2 = [
                        ['field1'=>'field1','field2'=>'field2','field3'=>'field3'],
                        ['field1'=>'field1','field2'=>'field2','field3'=>'field3'],
                        ['field1'=>'field1','field2'=>'field2','field3'=>'field3'],
                     ];
        $this->genreateXml($startTag1,$startTag2);
    }
    public function genreateXml($startTag1,$startTag2){

        $startTag1 = $this->xml->createElement('startTag1');
        $this->add_dom_child($startTag1, 'Field 1', $startTag1['field1']);
        $this->add_dom_child($startTag1, 'Field 2', $startTag1['field2']);
        $this->add_dom_child($startTag1, 'Field 3', '$startTag1['field3']');
        $this->xml->appendChild($startTag1);

        $startTag2 = $this->xml->createElement('startTag2');
        if(!empty($startTag2)){
        foreach ($startTag2 as $key => $startTag) {
            $startTag2NodeName = $this->xml->createElement('startTag2NodeName');
            
            $this->add_dom_child($startTag2NodeName, 'Field1',$startTag['field1']);
            $this->add_dom_child($startTag2NodeName, 'Field2', $startTag['field2']);
            
            $startTag2->appendChild($startTag2NodeName);
        }
         $this->xml->appendChild($startTag2);

    }
    $xmlString = $this->xml->saveXML();
    $xmlString = substr($xmlString, strpos($xmlString, 'startTag1'));
    /*Obejct to send with request*/
    $data_obj = new SoapVar($xmlString, XSD_ANYXML); 
        
    }
    private function add_dom_child (&$parent, $tag, $value, $type = 'text') {
        $node = $this->xml->createElement($tag); 
        $node = $parent->appendChild($node);
        $value = trim(preg_replace('/\s+/', '',$value));
        switch($type) {
            case 'cdata':
                $contxt = $this->xml->createCDATASection($value);
                break;
            case 'bool':
                $value = ($value === true) ? 'TRUE' : 'FALSE';
                $contxt = $this->xml->createTextNode($value);
                break;
            default:
                $contxt = $this->xml->createTextNode($value);
                break;
        }
        $contxt = $node->appendChild($contxt);
    }
}