Sounds like he wants to sell PHP programs in a format that the client couldn't reverse-engineer/resell them easily.
Short answer: No.
Long answer: Yes, but ...
Unless you are able to find a non-Zend compiler or host the files on a server which you have sole control, you don't have much of a choice but to trust your clients (standard procedure is to include a clause in the initial contract that protects against this).
However, there is another option.
A while back, I seem to remember someone on the board throwing together a script that renamed every variable to a random series of characters (unique to each variable, of course), removing every newline, toasting all comments and some other formatting stuff. This would make it pretty confusing for an end-user to play with. You would, of course, keep a version that you could play with and re-sell. This is not foolproof, of course, because the encoding script could be reversed and except for the variable names (which I generally find very helpful when trying to figure out what a program does), the original program could be reconstructed.
Short of this, I can't think of anything.
-Ben