FNV is a simple algorithm and produces good results.
I needed this functionality as part of another project that required automated generation of relatively large number of unique usernames based on information provided by the user.
Some challenges: Apparently PHP does not work well with numbers, especially large ones. I spent few hours trying to find out why my implementation was not producing correct results, … realized in the end that arithmetic with big numbers is something you should not do with PHP. To resolve these issues I had to use bitwise shift and add operations instead of multiply.
I put the code here: http://code.google.com/p/boyanov/source/browse/?repo=prototypes#hg/fnvhash.
… and a simple wiki page here: http://code.google.com/p/boyanov/wiki/FNVHash.
More information about the Fowler-Noll-Vo hash function could be found here:
Neven Boyanov's personal blog 
Hi – you define FNV_prime_32 but I can’t see where you multiply the offset using this prime in the example code.
Tom