I have a client who purchased a php software package, however the company they purchased the software from no longer supports the software. They have contracted me to modify there code. About 90% of the code is human readable where 10% is encoded using the Zend Encoder.
The main point of the Zend Encoder is to protect intellectual property in code by making it executable, but not readable. You won't be able to decompile it; you'll have to reverse-engineer it (if this is legal under applicable laws and your license). Most likely the bits which are most difficult to reverse-engineer alre also the ones that are encoded (at least this would be the most sensible way to offer a product that's only partially encoded). You may want to see if it isn't possible to offer to sign some kind of non-disclosure agreement with the company and get the source code, if they aren't supporting it themselves.