The code snippet below should replace the parentheses and whatever's inside them with the contents of the $replacement variable, unless I screwed up the regex. Check the php manual entry for preg_replace for a more detailed discussion of this function.
<?php
$string = 'something (more)';
$replacement = 'less';
$string = preg_replace('/\(*.\)/',$replacement,$string);
echo $string;
?>