Here's an example of an ISP restricting PHP to CGI on shared hosts, quoted from PHP Safe Mode, problems and work-arounds:

For complex security reasons, (mt) Media Temple's various hosting solutions operate PHP in what is known as "safe mode". This is necessary for the safe running of PHP enabled web sites in a shared server environment...

Safe mode imposes several limitations that might be causing your scripts to not function as anticipated...

Please note that many "ready-made" PHP scripts, such as Gallery or ***NUKE, use the system () function to copy files...

One known workaround on the (ss) Shared Server systems is to use an .htaccess file to force your php scripts to run in "CGI Mode"...

Customers requiring a hosting solution without the restrictions of PHP safe mode should consider (mt) Media Temple's other family of hosting products such as the (as) Appliance-Server, or (ds) Dedicated-Server.

"Most ISPs are offering PHP through either CGI or FastCGI (mod_php has all the same security issues mod_perl does)."-perrin