Parsing XML Response with xml schema - php

Question

Edit
The code given below will work in localhost as it is, if anyone want to copy and try it.The given credentials are valid.
I have a php code which requests data from an API service. An XML request is sent and in response XML data is recieved. I have stored the respose data in a variable $output. When doing echo $output, the details in the XML response is printed. But now I need to parse this response and store the required data in variables. E.g: I need to save $customer_id = value from the <customer_id>12345</customer_id>. I did a thorough google search and tried all the snippets provided by different developers, but no use.
I tried var_dump(simplexml_load_string($output)); and it is returning object(SimpleXMLElement)#1 (0) { }. I even tried converting the XML data to array.
index.php
<?php
$appId ="MFS149250";
$appPass ="5TEBRPCZ";
$brokeCode ="ARN-149250";
$iin = "5011217983";
$xml_data = '<?xml version="1.0" encoding="UTF-8"?>
<NMFIIService>
<service_request>
<appln_id>'.$appId.'</appln_id>
<password>'.$appPass.'</password>
<broker_code>'.$brokeCode.'</broker_code>
<iin>'.$iin.'</iin>
</service_request>
</NMFIIService>';
$URL = "https://uat.nsenmf.com/NMFIITrxnService/NMFTrxnService/IINDETAILS";
$ch = curl_init($URL);
curl_setopt($ch, CURLOPT_MUTE, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/xml'));
curl_setopt($ch, CURLOPT_POSTFIELDS, $xml_data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$output = curl_exec($ch);
//echo "<textarea>".$output."</textarea>";
echo $output;
var_dump(simplexml_load_string($output));
curl_close($ch);
?>