PHP4 and PHP5

[eluser]cahva[/eluser]
PHP 5 is mainly backwards compatible so you should be ok if you tend to code in todays standards(=dont use register_globals, dont use old $HTTP_POST_VARS etc.. In general, havent used deprecated functions).

If you start developing a new site, why start it in PHP4 server anyway? PHP 4 is mostly dead and the support has been dropped a long time ago.

CI itself will work if its a recent version. Check PHP changelog for changes regarding your own code. From version PHP 5.3 theres some dramatic changes and many deprecated functions are removed from PHP. Most webhosts(us included) still havent installed PHP 5.3 because of this and use 5.2.x.