[PHP] Quotes in enum values are not escaped

Description

If enum values contain single quotes the codegen generates not valid PHP code.

Swagger-codegen version

2.4.0

Swagger declaration file content or url

{
  "swagger":"2.0",
  "info": {
    "title": "Foo",
    "version": "1.0.0"
  },
  "paths": {
  },
  "definitions": {
    "Foo": {
      "type": "object",
      "properties": {
        "type": {
          "type": "string",
          "enum": [
            "Foo 'bar'"
          ]
        }
      }
    }
  }
}

Command line used for generation

docker run --rm --volume=$PWD:/app --workdir=/app swaggerapi/swagger-codegen-cli generate \
    --input-spec ./swagger.json \
    --lang php

Generated PHP-file contains syntax error

<?php

namespace Swagger\Client\Model;

class Foo implements ModelInterface, ArrayAccess
{
    ...

    const TYPE_FOO_BAR = 'Foo 'bar''; // unescaped quotes

    ...
}

Start by running the documented Docker generation command with the provided Swagger definition and PHP output. Trace the PHP generator entry point and the template that emits enum constants, then verify that the generated Foo class contains valid escaping for the quoted enum value and can be parsed as PHP.