If you're running PHP5 on shared hosting through GoDaddy you will want to create a php5.ini file in your root directory and copy the contents of your php.ini file into it. Apparently GoDaddy doesn't do this for you.
I ran into this issue because I needed to add the zend extension that pointed to the folder where ionCube was located after I installed s2member-pro. I originally tried changing the php.ini file but the GoDaddy server wouldn't recognize the update. There is a post about it here:
http://help.godaddy.com/topic/320/article/5647#linux
Once I created the file and added the zend extension (I modified the path to reflect what folder ionCube was located in) my installation of s2member-pro was error free.
-Cheers