+ `<?php`
  + keywords
  + function/method names
* updates to the language:
  + support `_` in (binary, decimal, hexadecimal, ...) numbers (7.4.0)
  + support for binary numbers (`0b...`) (5.4.0)
  + Unicode codepoints escape syntax (`\u{...}`) (7.0.0)
  + add some missing keywords:
    * `fn` keyword (7.4.0)
    * type declarations (PHP 7, including `void` and nullable types from PHP 7.1)
    * 7.0.0: `class` (anonymous classes), `yield from`
    * 5.4.0: `callable`, `insteadof`, `trait`, `__TRAIT__`
    * 5.3.0: `goto`, `__NAMESPACE__`, `__DIR__`
    * others: casts, `instanceof`, `__CLASS__`, `__FUNCTION__`, `__METHOD__`, `__halt_compiler`
    * `self` even if it's not really a reserved word

we already had a rule to handle their lower case form,
just make it case insensitive.

* remove E_* and PHP_* constants: these are not keywords and are case sensitive
(keywords are not)
* `not` doesn't exist
* neither `this`
* `empty` should be treated in the same way as `isset` and `unset` (currently functions)
* `virtual` is a function, not a keyword
* `php_user_filter` is a class, not a keyword

…n PHP

typehinting should be placed before the one whith `?` and which
recognizes, among others, operators like `?`, `??`, `??=`, ...

In clear, `?int` is actually tokenized as:
`['Operator', '?'], ['Keyword.Type', 'int']`
Instead of:
`['Keyword.Type', '?int']`