You may get the following error in the server error log :
PHP Warning: include(DOMDocument.php) [<a href="function.include">function.include</a>]: failed to open stream: No such file or directory in ......
PHP Warning: include() [<a href="function.include">function.include</a>]: Failed opening 'DOMDocument.php' for inclusion (include_path='.....
PHP Fatal error: Class 'DOMDocument' not found in .....
This error is because php-xml module is missing. To solve the issue install the php-xml # yum install php-xml