Most of the syntax that is used in PHP is derived from C. You can start with C++, which is what I did, but I went back and studied C afterwords. It is easier to learn C first, which would build a good foundation for learning C++.
If you are in a situation where you would program in both languages, I would definately recommend doing C first. If you learn C++ first, then try to do C you will find yourself getting a little irritated at some of the things that you can't do that you take for granted in C++. Like input/output functions. Whereas you are already use to using functions like printf, sprintf for output (which are C functions), you can still use these functions in C++, but C++ has a I/O stream library so that you can use cin and cout for input and output. This is only for C++ though.
By learning C first, you only gain programming power by moving up to C++ afterwords. If you do the reverse, you only feel more restricted.