PHP doesn't actually need the definition of the class to know its fully-qualified name - all the required information is available at compile-time. So it doesn't try to load it.

I understood you perfectly, however this subject were raised by several different people debating about it and I would like to share this knowledge with them. So, would you mind to provide a little more elaborated version of this explanation?

This isn't really an appropriate place for a discussion so I'll keep this brief. Try a mailing list or forum for something more in-depth.
http://www.php.net/support.php

Namespaces and uses are resolved at compile-time (ie, when PHP "compiles" the file before execution). That's why there are strict requirements about how they can be used. Because of those requirements, when PHP encounters a class name it can immediately know the fully-qualified name. Think of it as a filesystem: the namespace is like a directory for relative locations and the use statements are like symlinks.

The class name is either absolute ("\Testing\Test") or relative ("Test"), and if relative it could be a normal name

  namespace Testing {
    echo Test::class; // \Testing + Test = \Testing\Test
  }

or an alias

  use Testing\Test as AliasedTest;
  echo AliasedTest::class; // AliasedTest + use = \Testing\Test

Remember autoloading? Without all this, autoloading wouldn't work!

::class is just a new tool to expose information PHP has always known.

Thank you very much. Your time clarified one point for two different communities.