Is it better to use require_once('filename.php') or require_once 'filename.php';
Pear Coding Standards say :
"include_once and require_once are statements, not functions. Parentheses should not surround the subject filename."
Source : http://pear.php.net/manual/en/standards.including.php
It's exactly the same thing. It's a matter of style.
The parentheses may get in the way some times. For instance, this example from the manual doesn't do what you expect:
if (include('vars.php') == 'OK') { echo 'OK';}See example #4.