require_once doesnt work well in vhosts

[rednimer]

Im trying to move my website from a server without plesk to one with plesk. The problem I seem to be having is i have require_once commands throughout the script, and it doesnt seem to work right. If I change it to require_once'/var/www/vhosts/domain.com/httpdocs/filename.php' I can get it to work, but I shouldnt have to do that. I should be able to leave it as require_once'file.php'.

And who knows if its messing up other commands also.

Anyone having the same problem, or know how to resolve it?