you mean encrypt php source code? If thats what you are looking for, then you have to use ZendGuard or Ioncube for code obfuscation.
Not to sound all Confuscious on you, but obfuscation is in you, not the code. obfuscate: to make obscure or unclear So, to the really only way to make things clear, is to get in there, parse the code into a manner you will understand. This means working through it line by line, adding indentations, spaces, line breaks as needed to see the actual code structure. From there, search and replace variable, function and class names into names that offer more meaning to you. As you do both of those and you gain understanding of what each line does--comment it.
I think he's (or she) asking in laymen terms, how to crack a script encoded with Zend Guard, IonCube Encoder, and so forth.