Long integer to 16 character Hex for Google Reader sync

Python Code

h = "%s"%("0000000000000000%x"%(i&0xffffffffffffffff))[16:]

I'd like to know how this would be done in PHP.

The application is for google reader syncronization. An initial sync list comes with numeric values, but the actual article id hex.

Example Long Integer = 8643979098044673995

Example Google Reader Article ID = tag:google.com,2005:reader/item/77f5953120daafcb

sprintf("%016x", 8643979098044673995);