I decode some of my ionCube encoded PHP pages but it contain code with obfuscated PHP function names:
Obfuscation is designed to be irreversible - if there was a method to easily reverse the obfuscation then using obfuscation would be pointless. Think of it as hashing the function name -- although it may be possible to bruteforce the original names, you'll have to put some serious effort into it, including:
As indicated in the comments, your only realistic chance at retrieving the original source code is to contact the original developer. You wouldn't expect to gain the original source code from a binary file compiled from C code, would you?