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Description
Is your feature request related to a problem? Please describe.
It would be good to have an option to generate interfaces at least for API classes. That's related to all current PHP clients. Modern PHP development use interfaces as typehints instead of precise class names because it gives more flexibility with dependency autowiring.
Describe the solution you'd like
When generator config option includeInterfaces=true then output contains interface for each API class, like UsersApi -> UsersApiInterface.
Describe alternatives you've considered
We can add that option to the php-nextgen generator only.
Additional context
Can do myself and submit PR. I would add that config option at root level to AbstractPhpGenerator. Template for the interface should be pretty easy, because it obviously contains methods declarations only.