Hey,
I found what was happening.
Basically, when installing PHP in my computer for developing, I was following some instructions that said to copy all DLL files in the 'ext' folder (PHP 5) to the Windows folder. But, then in other guide I saw that is was unnecessary so I removed all those files from the Windows folder.
Here is the funny thing (it actually took me an entire day to find out about this), when I restarted the computer and run again (the first time I didn't see the warning about a missing php_domxml.dll at startup) mysteriously my code stopped working.
It was until I restarted my computer again (several hours later) that I saw that message again. So I copied the file php_domxml.dll again to Windows folder and all worked perfectly (bad for me I already replaced all my code by simple_xml).
So, what happened is that the library got loaded when I copied it to the Windows folder, and everything worked until I restarted the computer and PHP could not find the DLL file that was removed.
Everything can be fixed by just copying the file php_domxml.dll to the Windows folder.
Sorry the long answer and by the multiple posts.