PHP4 and PHP5

[eluser]cahva[/eluser]
Every library and helper in userguide will work in PHP4/PHP5 server.

If your site is moved to PHP5 server(hope they dont just upgrade the PHP from version 4 to 5, thats just asking for trouble), it will work(as long as you have upgraded to latest CI). Theres really no bad things when upgrading PHP4 to PHP5. Some ancient/badly written code might not work straight in PHP5, but usually does and is pretty trivial to get working.

And you can make sure that the site works with PHP5 by asking your host to setup webspace and temporary address for you to test it out. And if it does, change the site to point to the new server. This is the usual process for us(I work in a hosting company) to make sure the site works in the new server. If your host dont setup you a webspace with temporary address, they are dicks