I found a good question asked by new babie on PHP in netpro discussion. The question was, "According to PHP concept, it should happend at each and every hop."
The answer for the question is simple and straight forward. Always keep in mind one thing that PHP happens only for the destined network not for the intermediate routers. So only that router will perform the PHP which will become the second last router of destination in service provider cloud, rest all the routers will perform the swapping.