legal issues?

If I design and code a dynamic site for a client, which uses php and mySQL, and it (the site) is going to be used on his server, am I the one who has to worry about licensing or is he?

It is him right? Because he will be paying me to design and script his site...

Or am I wrong?

If you are really concerned about this, talk to a lawyer in your area that is familiar with your local state laws. I am not a lawyer, so do not take this as anything other casual advice.

In short, if it's not explicitly specified as to who owns the code and who is responsible for licensing, you both could probably make convincing arguments that it's the other guy's problem. If it's his server though and all you are being hired for is to supply code, it should be pretty easy to have a contract specify that he is responsible for all licenses for software running on his server.