Make behavior of PHP and GitHub references consistent

[wouterj]

Changes

• Make escaping consistent for PHP and Github references (require double backslashes, as single can be used to escape the ` character).
• Add namespace support to :phpmethod: (consistent with :method: and :phpclass:)
• Update PHP reference text/title to be consistent with GitHub references (e.g. strip the full namespace & only show method name)
• Use String component in all reference classes

The updates should be fully compatible with Symfony documentation. symfony/symfony-docs#20822 is the only case where we use the new namespace features of :phpclass: (and this revealed the inconsistency).

[javiereguiluz]

Very nice! Thanks Wouter.