Original mentioned in: #1024 (comment)

The PHP top level namespace is reserved for PHP itself.

I've been wondering whether we should add a sniff for it and am leaning towards "yes" as, even though it is currently not used in PHP itself, there has been discussion about starting to use it.

This is on par with the warning about double underscore prefixed function names, as that is also reserved for PHP itself.

Code samples:

// Ok.
namespace MyPHPApp;
namespace My\PHP\App;

// Warning.
namespace PHP;
namespace PHP\App;