Description:
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The documentation is missing the fact that the following statement is possible :

function o1(Foo  $arg0 = null, $x) {}
function o2(Foo& $arg0 = null, $x) {}

The code above works on 5.2.3 (which is fine).

The fact that a typehinted parameter can have a default (enforced to null) value should be explained at least in the type hinting part of the doc [http://ch2.php.net/manual/en/language.oop5.typehinting], with the version of php that support it (I can't remember it it was  working on 5.1).

Note also that the syntax can be misleading: one can easily think that $arg0 take a default value, and that o1('a') is possible (or o1(,'a')?).