How to retrieve information from soap response?

Hi,

I'm trying to make a soap call to an api and use a piece of information from the response I get.
Below is the code I use, the part where I make a soap call works fine, I get a correct response from the server but here comes the problem I have, I can't retrieve the information I need from the response.
The response contains an ID between the response-tags I need for a database insert.

I tried retrieving information with simplexml_load_string but that didn't work, so now I'm trying it with xml_parse_into_struct.
I use this in the code below and I see the information when I use print_r but I can't seem to access it.

Can anybody tell me how to access the information or show me an other method to retrieve this information from the response?

I'm new to PHP so maybe I'm asking a very simple question but I can't seem to get it right or find a straight forward answer to my problem anywhere.
Thanks in advance!

PHP:
<?php
	$client = new SoapClient('https://api.tripolis.com/soap/1.5/DialogueService?wsdl', array('trace' => true, 'exceptions' => false));
 
	//piece of code that creates $params
 
	try
	{
		$client->upsertContact($params);
 
		$response_string = $client->__getLastResponse();
 
		//removes headers from response
		$start = strpos($response_string, '<soap:Envelope');
		$end = strrpos($response_string, '>');
		$response_string = '<?xml version="1.0" encoding="utf-8"?>'. substr($response_string, $start, $end-$start+1);
 
		$simple = $response_string;
		$p = xml_parser_create();
		xml_parse_into_struct($p, $simple, $vals, $index);
		xml_parser_free($p);
 
		echo 'Index array\n';
		print_r($index);
		echo '\nVals array\n';
		print_r($vals);
 
		exit();
	}
	catch (SoapFault $fault)
	{
		echo '<hr>At line number: '.__LINE__.' SOAP Fault: (faultcode: {$fault->faultcode}, faultstring: {$fault->faultstring})';
		echo '<hr><pre>';
		var_dump($client->__getLastRequest());
		echo '</pre>';
 
		exit();
	}
?>
 
XML:
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
	<soap:Header>
		<api-version>1.5.9.1</api-version>
	</soap:Header>
	<soap:Body>
		<ns2:upsertContactResponse xmlns:ns2="http://api.tripolis.com/">
			<response>Mjg1Jhe03MjCMdCDprzC_0ZQpPqczsOvM</response>
		</ns2:upsertContactResponse>
	</soap:Body>
</soap:Envelope>
 
xml_parse_into_struct:
Index array
Array
(
    [SOAP:ENVELOPE] => Array
        (
            [0] => 0
            [1] => 9
        )
    [SOAP:HEADER] => Array
        (
            [0] => 1
            [1] => 3
        )
    [API-VERSION] => Array
        (
            [0] => 2
        )
    [SOAP:BODY] => Array
        (
            [0] => 4
            [1] => 8
        )
    [NS2:UPSERTCONTACTRESPONSE] => Array
        (
            [0] => 5
            [1] => 7
        )
    [RESPONSE] => Array
        (
            [0] => 6
        )
)
 
Vals array
Array
(
    [0] => Array
        (
            [tag] => SOAP:ENVELOPE
            [type] => open
            [level] => 1
            [attributes] => Array
                (
                    [XMLNS:SOAP] => http://schemas.xmlsoap.org/soap/envelope/
                )
        )
    [1] => Array
        (
            [tag] => SOAP:HEADER
            [type] => open
            [level] => 2
        )
    [2] => Array
        (
            [tag] => API-VERSION
            [type] => complete
            [level] => 3
            [value] => 1.5.9.1
        )
    [3] => Array
        (
            [tag] => SOAP:HEADER
            [type] => close
            [level] => 2
        )
    [4] => Array
        (
            [tag] => SOAP:BODY
            [type] => open
            [level] => 2
        )
    [5] => Array
        (
            [tag] => NS2:UPSERTCONTACTRESPONSE
            [type] => open
            [level] => 3
            [attributes] => Array
                (
                    [XMLNS:NS2] => http://api.tripolis.com/
                )
        )
    [6] => Array
        (
            [tag] => RESPONSE
            [type] => complete
            [level] => 4
            [value] => Mjg1NjgzMjCMdCDprzC_oZ8rP*czsOVN
        )
    [7] => Array
        (
            [tag] => NS2:UPSERTCONTACTRESPONSE
            [type] => close
            [level] => 3
        )
    [8] => Array
        (
            [tag] => SOAP:BODY
            [type] => close
            [level] => 2
        )
    [9] => Array
        (
            [tag] => SOAP:ENVELOPE
            [type] => close
            [level] => 1
        )
)

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