If you want that other programmers would not be able to read your code, then I will be the bad messeger: there is no way to encrypt your source code - as it must be run by php interpreter, this means that it must be decrypted at some point. So any advanced programmer could intercept this code at this stage (after code is decrypted and before it comes to interpreter). So encryption only adds slowlines for your code and gives nothing usefull.
There is one exception - if you write your own php language interpereter then you will be able to run your code on your server. If you want to run php code on customer's server or on any shared hosting then there is no way to encrypt it.
But obsfucation would be more interesting to you. Obsfucated code is not encrypted, but it is transformed so, that it is impossible to human to read code (spaces removed, function names changed to any random names/letters, etc.). At most cases it will be enough.
P.S. By the way, please think if you are realy writing so unique and valuable code that it is worth stealing. Most programs and needed functions are already written and open sourced, any programmer can use it.