[PHP] add AbstractPhpCodegen to be as the base class of PHP-related generators

[abcsun]

No description provided.

[wing328]

For #3227

[wing328]

cc @arnested

[wing328]

I think apiDocPath and modelDocPath are still used in PHP API client documentation (markdown) so we should keep it.

[wing328]

@abcsun when you've time, can you help do a rebase on the latest master to resolve the merge conflicts?

[abcsun]

@wing328 please review

[wing328]

I did some tests with PHP API client, Slim, Silex, Lumen and found 1 issue.

If I run the following command:

   java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate \
   -i modules/swagger-codegen/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml \
   -l lumen \
   -o /var/tmp/lumen/ 

it shows the following error:

[main] WARN io.swagger.codegen.languages.AbstractPhpCodegen - Return (reserved word) cannot be used as model name. Renamed to ModelReturn
[main] WARN io.swagger.codegen.languages.AbstractPhpCodegen - Return (reserved word) cannot be used as model name. Renamed to ModelReturn
[main] ERROR io.swagger.codegen.AbstractGenerator - null/api.mustache (No such file or directory)
[main] ERROR io.swagger.codegen.AbstractGenerator - can't load template null/api.mustache
Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Fake'
    at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:478)
    at io.swagger.codegen.cmd.Generate.run(Generate.java:223)
    at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:36)
Caused by: java.lang.RuntimeException: can't load template null/api.mustache
    at io.swagger.codegen.AbstractGenerator.readTemplate(AbstractGenerator.java:50)
    at io.swagger.codegen.DefaultGenerator.processTemplateToFile(DefaultGenerator.java:643)
    at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:437)
    ... 2 more

./bin/lumen-petstore-server.sh works fine as it's using -tmodules/swagger-codegen/src/main/resources/lumen`.

@abcsun please let me know if you need help fixing the issue.

[abcsun]

@wing328 Forgot to set the templateDir in LumenServerCodegen, it has be fixed.

[wing328]

@abcsun thanks for the fix. It works for me now.