Both str_ireplace and simplexml_load_file are built in functions for PHP5. You said you have PHP5 installed, so the only other thing it could be is your host might have these functions disabled. Can you post a link to your php info page you made so we can see what settings it lists?
The fix is going to be working with your host and having them make sure PHP5 has the standard functions it normally should. PowerGallery requires a normal PHP5 installation to work properly.