Apache Pre-Processing/Mod Rewrite Script

I need a portion of a VERY simple cloaking/IP delivery script to be created for me. Here's how it must work: For each visitor to my website, determine if it is a search engine spider or not. If the visitor to my website is a search engine spider, your script will take note of the file name being requested, the keyword associated with the spider's visit, and the search engine from which the spider is visiting. This data will be passed to an external PHP script, which will return the name of the file to be served to the spider. The spider must NOT be forwarded to a different page; rather MOD-REWRITE must be used such that the new content selected by the external PHP program is served to the spider as if that content was contained in the file originally requested by the spider. If the website visitor is not a SE spider, then allow the request to be handled as normal under Apache. Note that you do NOT need to create the external PHP script mentioned above, though you will need to provide documentation on how to call an external PHP script from within your script. This is a fairly simple thing If you know what you're doing. Eager to get moving on this...