Description:
------------
---
From manual page: https://php.net/language.types.declarations
---

If you use those pseudo-types to define a parameter|variable|UnionType, it'll assume a String value containing a valid Class|Interface|Trait full qualified name only if it passes through their respective *_exists functions. It can also assume an object as value.

Otherwise a runtime exception must be thrown because that no class, interface or trait were defined with the String value as name, even after autoload performance.

This feature can be further discussed.

Test script:
---------------
<?PHP

fn ( Class|Interface|Trait|Object $Subject )
{
   if ( is_string($Subject) )
      echo 'The parameter $Subject is a valid Class|Interface|Trait name.';
   elseif ( is_object($Subject) ) 
      echo 'The parameter $Subject is an object of any class.';
   elseif ( $Subject === null )
      echo 'The parameter $Subject is empty.';
   else
      echo 'This code line will never run.';
   echo PHP_EOL;
}

fn ( new StdClass ); // Works.
fn ( function () {} ); // Works.
fn ( 'Defined' ); // Works.
fn ( 'Undefined' ); // Throws some runtime exception because there's no class, interface or trait defined with that name.

Expected result:
----------------
The parameter $Subject is an object of any class.
The parameter $Subject is an object of any class.
The parameter $Subject is a valid Class|Interface|Trait name.
RuntimeException: Invalid parameters content.