PHP error after upgrading from php 5.3 to 5.6 - php

Question

I had this line of code which was working fine until I update the php version of my server from 5.3 to 5.6.
It re-writes a xml file.
I guess this line is not compactible with 5.6, but I don't know what is wrong exactly:
$newXml->asXml('my_xml.xml');
And this is the error message:
Fatal error: Call to a member function asXml() on boolean in /usr/home/example/www/sites/all/modules/custom/inmovilla_rewrite/inmovilla_rewrite.module on line 71
What changes should I do here?
UPDATE Complete function
<?php
/**
* Implemetns hook_cron()
*/
function inmovilla_rewrite_cron() {
$xml_external_path = 'http://ap.apinmo.com/portal/mls/jamp623433/my_xml.xml';
$xml = file_get_contents($xml_external_path);
$pattern = '/<unico>(.*?)<\/unico>/';
$response = preg_replace_callback($pattern,function($match){
$valueUnico = trim($match[1]);
$valueUnico = substr($valueUnico, 0, -4);
return '<unico>'.$valueUnico.'</unico>';
},$xml);
$searches = array();
$replacements = array();
for ($i = 1; $i <= 30; $i++) {
$searches[] = 'foto'.$i.'>';
$replacements[] = 'foto1>';
}
$response = str_replace( $searches, $replacements, $response ) ;
$newXml = $response;
$newXml = simplexml_load_string( $newXml );
$newXml->asXml('my_xml.xml');
}

Answer 1

You are calling the method: asXml() on a boolean data-type, not an instance of SimpleXMLElement - you are presumably not feeding in a "well-formed" XML string.
See here: http://php.net/manual/en/function.simplexml-load-string.php
Returns an object of class SimpleXMLElement with properties containing the data held within the xml document, or FALSE on failure.
You should firstly implement some error checking rather than assume that you have a SimpleXMLElement instance, e.g.
$newXml = simplexml_load_string($newXml);
if ($newXml instanceof SimpleXMLElement) {
$xml = $newXml->asXml('my_xml.xml');
}
However, you will need to do some further debugging on the values held in $newXml / $response to see what is wrong with the XML string. I cannot see the XML contents in your question to advise any further as to why the change of PHP version has affected this.