Magento add debug trace in system.log? - php

Question

This error keeps filling system.log:
2016-03-27T13:59:10+00:00 ERR (3): Warning: simplexml_load_string(): Entity: line 12: parser error : Premature end of data in tag config line 2 in /home/xxxx/public_html/lib/Varien/Simplexml/Config.php on line 510
Every a few minutes or a few hours. To find the culprit, I tried to add this in Config.php:
file_put_contents('/home/xxxx/public_html/xml_errors/'.sha1(microtime(true)).'.txt', $string . PHP_EOL . '=====' . PHP_EOL . print_r(debug_backtrace(), true));
Before the 510 line of Config.php:
$xml = simplexml_load_string($string, $this->_elementClass);
Which is very quickly creating 5 - 10 files per second as my store is pretty busy and I still don't know how to pinpoint the $string that actually raises the warning.
How can I customize system.log to give debug trace of the warning so I can trace the actual source of the xml string that was passed to Config.php?
Or any other suggestion to find the source of the invalid XML would be very much appreciated!

Answer 1

I was able to trace back by adding to the problematic line:
$xml = simplexml_load_string($string, $this->_elementClass);
Like this:
set_error_handler("warning_handler", E_WARNING);
$xml = simplexml_load_string($string, $this->_elementClass);
restore_error_handler();
And then add a standalone function at the end of the Config.php file:
function warning_handler($errno, $errstr, $errfile, $errline) {
file_put_contents('/home/xxxx/public_html/xml_errors/'.sha1(microtime(true)).'.txt', print_r(debug_backtrace(), true));
}
You must manually create the folder xml_errors of course.