I personally use apache. I wish to let php4 exist together with php5 in a single server, but what type of method maybe there is?
You will find a lot of guides regarding how to do that:
- Running Apache2 With PHP5 And PHP4 At The Same Time
- Running both PHP4 and PHP5
- Configure Apache to work with PHP4 and PHP5
- Running PHP4 and PHP5 in parallel
... and more
What I have seen hosts do would be to keep interpretation .php files as PHP4, but use .php5 for PHP5.
My host allows me put b .htaccess having a special directive to express 'process php pages within this folder and subfolders as PHP5'
Request your host, they might have something similiar.