There are plenty of free script out there that will rotate banners.. But most will do it per page refresh..
Search for "simple php code to rotate banners" for php code to accomplish your task..
There is phpads:
PHP Ad Manager | Get PHP Ad Manager at SourceForge.net and OpenX:
OpenX (phpAdsNew) | Get OpenX (phpAdsNew) at SourceForge.net
It really depends on what you are doing.. I would say you prob need something pretty simple.