Your xpub should be the extended master public key. Why do you believe that your derivation path is formed according to BIP44?
I don't see any clue for that in your code. Is that somewhere implicitly used in the modules of that Bit-Wasp/bitcoin-phpLibrary?

Probably you'd need to dig in the code there further. You will have to find out what full derivation path for a client user address is used.

To narrow it down, print some log line for $pubPrefix, $config and add some log lines for function derivePath(), maybe you can see there how the derivation path is assembled.