[PHP] Remove NUMBER_ prefix from enum vars if a name is provided, show enum descriptions

[JulianVennen]

You can provide names for integer enums using x-enum-varnames. When generating a PHP client using this property the enum names are still prefixed by NUMBER_.
If your enum name is ONLINE it will be NUMBER_ONLINE which is quite odd.

This PR fixes that by only adding the NUMBER_ prefix when the name provided starts with a digit.
It also fixes issues with other values of x-enum-varnames containing spaces or other forbidden characters.

Providing a description for each enum value can be done with x-enum-descriptions which is now also supported.

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@jebentier, @dkarlovi, @mandrean, @jfastnacht, @ybelenko, @renepardon

[wing328]

thanks for the PR

tests passed locally

PHPUnit 9.6.19 by Sebastian Bergmann and contributors.

...............................................................  63 / 222 ( 28%)
............................................................... 126 / 222 ( 56%)
............................................................... 189 / 222 ( 85%)
.................................                               222 / 222 (100%)

Time: 00:01.442, Memory: 12.00 MB

OK (222 tests, 380 assertions)

[wing328]

sorry my tests were not done in the right way

i just re-run the tests but got

Time: 00:01.243, Memory: 12.00 MB

There was 1 error:

1) OpenAPI\Client\EnumTestTest::testPossibleValues
Error: Undefined constant OpenAPI\Client\Model\EnumTest::ENUM_INTEGER_1

/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/tests/EnumTestTest.php:14

can you please update the tests accordingly? thank you