php包括同一服务器上从一个目录到另一个目录的文件

I have several instances of a CMS on the same serve and plan to have a main hub where I can update the majority of the files which I want to include into each CMS. I was directed to symlinks by my hosting company of which I tried a test page e.g:

symlink('/home/site1/public_html/includes', '/home/site2/public_html/includes');

and tried to print out something from the site1 into site2 but I got an error

Warning: symlink(): No such file or directory in...

I have also in desperation tried to do a basic include from site one i.e

include('/home/site1/public_html/includes').

I know I may be way off which is why I posted on here.