I've developed an application in PHP - Codeigniter. I would like to sell that to clients who are interested. Application has to be hosted on client server.
But, he are comes the questions
My whole point is even after the product sold to someone it's maintenance should be done by me and it shouldn't replicate.
Any process / application can help on this to encrypt the complete project and runs the application in normal way without any hassles in local / server.
There are some encrpytion frameworks for php such as ZendGuard and Ioncube. I used both for different projects and they are both very capable. However, ioncube is more up to date and supports newer php versions. You can encode your project with a few options including mac address or domain restrictions etc. Those softwares also use opcode cache so they can even run your code slightly faster if you already dont use any opcode cache.
You can use IonCube, but beware you can find decoders online pretty easily. So if someone wants to get your source code, they can.
Maybe you can look into code obfuscation in combination with ioncube. To make it a lot harder and the source code less usable.