Besides using NAT for accessing the internet with multiple machines using a single IP address, there are many other uses of NAT. One of them is to forward all traffic that is sent to a certain TCP port to another host. In practice, this technique can be used to test a service on a new host without adjusting anything on the client. The users or the clients do not need to be pointed to a new machine in order to test it. When the test would be unsuccessful, removing the NAT-rule is all it takes to switch back.
N.B. PHP 5.3 has not been supported now for almost 3 years, the best approach really would be to port any code over to a newer version, as the provided versions in EA4 are having fixes backported into them.