> it should say that an exception to this are objects, which are
> passed by reference.

Note, though, that this would still be misleading.  Consider

  function foo($object) {
      $object = null;
  }

vs.

  function foo(&$object) {
    $object = null;
  }

Only the latter has an effect which can be observed outside the
function; see <https://3v4l.org/04vX5>.

The "objects are passed by reference" statement is directly addressed in the OOP docs.

http://php.net/manual/en/language.oop5.references.php
> One of the key-points of PHP 5 OOP that is often mentioned is that "objects are passed by references by default".
> This is not completely true. This section rectifies that general thought using some examples.

The terms "by value" and "by reference" refer to the concept of value-types and reference-types.
https://en.wikipedia.org/wiki/Value_type
https://en.wikipedia.org/wiki/Reference_type
Those terms are being used correctly, but because PHP also has references the "by reference" is ambiguous.

The line on the Assignment Operators page is the faulty one.

Assignment of objects is by value as well (an object's value is the object itself*; that value is the one that is assigned).

What assignment does not do is create a new object with the same properties as the object being assigned (that's what clone is for).

* Which is why objects are said to be passed by value to functions. Since the value of an object is the object itself, changing the passed object's properties changes the passed object's properties.