Se quiseres para flashs:
# <?
# $banners = array("flash1.swf", "flash3.swf", "flash3.swf","flash4.swf");
#
# function randon($valor){
#
# srand((double)microtime()*1000000);
# $select=rand(0, count($valor)-1);
# $banner=$valor[$select];
#
# return $banner;
#
# }
#
# //Aqui vem o código do FLASH
# //para GIF animado é do modificar abaixo
#
# echo "<object classid='clsid:D27CDB6E-AE6D-11cf-96B8-444553540000' codebase='http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=5,0,0,0' width='300' height='50'><param name=movie value='".randon($banner)."'><param name=quality value=high><embed src='http://www.webtuga.com/forum/".randon($banners)."' quality=high pluginspage='http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash' type='application/x-shockwave-flash' width='300' height='50'></embed></object>";
# ?>
Normal e mais funções:
Ver aqui tutorial!