Random thoughts about `::class` in PHP

The special ::class constant allows for fully qualified class name resolution at compile, this is useful for namespaced classes.

I’m quoting the PHP manual. But things can be funny sometimes. Let’s go through some examples.

• use A\B as C;
  
  $_ = C::class;

  resolves to A\B, which is perfect 🙂

• class C
  {
      public function f()
      {
          $_ = self::class;
      }
  }

  resolves to C, which is perfect 😀

• class C { }
  
  class D extends C
  {
      public function f()
      {
          $_ = parent::class;
      }
  }

  resolves to C, which is perfect 😄

• class C
  {
      public static function f()
      {
          $_ = static::class;
      }
  }
  
  class D extends C { }
  
  D::f();

  resolves to D, which is perfect 😍

• 'foo'::class

  resolves to 'foo', which is… huh? 🤨

• "foo"::class

  resolves to 'foo', which is… expected somehow 😕

• $a = 'oo';
  "f{$a}"::class

  generates a parse error 🙃

• PHP_VERSION::class

  resolves to 'PHP_VERSION', which is… strange: It resolves to the fully qualified name of the constant, not the class 🤐

::class is very useful to get rid off of the get_class or the get_called_class functions, or even the get_class($this) trick. This is something truly useful in PHP where entities are referenced as strings, not as symbols. ::class on constants makes sense, but the name is no longer relevant. And finally, ::class on single quote strings is absolutely useless; on double quotes strings it is a source of error because the value can be dynamic (and remember, ::class is resolved at compile time, not at run time).