I never used any obfuscator, and wondered if it's possible to obfuscate for example a single function in a PHP file with Ioncube, and keep the others non-obfuscated in the same file?
ionCube is a compiler rather than a source obfuscator, and it is not possible to protect a single function as bytecode and source code cannot be mixed within a single file. The way to approach this would be to refactor your code so that the single function you want to protect is in its own file. If the function is a method, you provide a delegate method, or try having the method in a new class within the class hierarchy that is protected.