Add support for UUIDs in PHP

[pengpixel]

For PHP the UUID is already mapped as a string, but had no implementation yet and threw an error. If you have a schema which includes UUIDs and want to generate code for e.g. C# and PHP, the latter will fail. Giving you the choice to either maintain 2 schema files or just use strings for UUID values.

So this PR will:

• keep the string mapping and resolve the todo by validating the value with a regex, as PHP does not have built-in support for UUIDs