The problem here is likely to be a confusion about the meaning of "root directory."
Absolute path means exactly that -- relative to the operating system's root, not the Web server's notion of document root. If you are including /abc.php and the file is in /home/httpd/html/, PHP will not find the file.
Try this:
include($DOCUMENT_ROOT . "/abc.php");
You also might want to read the documentation for php.ini, where it's possible to fiddle around with include path defaults.