You could have a look for 'man ln' in Google which shows you how to link to other files in other folders.
But I should think that your PHP will be available to anyone in your group. This usually means your main account and any users/domains added since they all get assigned the same group unless you manually assign a new group.
So perhaps you just need an ini_set instruction in your PHP files.
I have my own installed PEAR version and I share it out by adding.
ini_get( 'include_path' ) . PATH_SEPARATOR . "/home/.//pear/php"
into the required PHP script.
That would be a pain to do for a lot of scripts so I would presume an entry in each domains .htaccess file may be possible. Hopefully someone will be along shortly to assist.
Using the pathnames,like what I do, is not really the best way btw since the server names can change so you may want to use $SERVER['DOCUMENTROOT'] instead.
Try a forum search I am sure this has cropped up before once or thrice.
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