Do you mean that you want to have PHP in your post itself, or in a theme page, or what?
If you want executable php in a post, you need the Exec-PHP plugin from here: http://www.soeren-weber.net/post/2005/08/18/50/
Thanks Otto42. I want to use a php include inside a page-post.
If I click on the link for the plugin, how do I get it to my server?
dstoops: It’s a plugin. Download it, extract the plugin’s files, ftp them to the plugins directory on your server, go to the admin page, activate the plugin. There may be more to it than that, but that’s pretty typical. Read the plugin’s webpage closer for install instructions.