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Doc Bug #20760 strange behaviour regarding $this
Submitted: 2002-12-02 04:24 UTC Modified: 2004-09-18 11:11 UTC
From: krennmair at webdynamite dot com Assigned: didou (profile)
Status: Closed Package: Documentation problem
PHP Version: 4.2.2 OS: RedHat 8.0
Private report: No CVE-ID: None
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From: krennmair at webdynamite dot com
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 [2002-12-02 04:24 UTC] krennmair at webdynamite dot com 
We have found a strange behaviour regarding $this. One of our programmers made a mistake during programming, which led to "Heisenbugs", which were not quite easy to find and fix. We could reduce the problem to a simple program to present it:

<?

class Foo {

  var $bla;

  function quux() {
    $this->bla = 5;
  }

}

class Bar {

  var $bla;

  function do_stuff() {
    $this->bla = 10;
    Foo::quux();
    echo $this->bla;
  }
}

$blabla = new Bar;

$blabla->do_stuff();

?>

The output is: "5"

Obviously, Bar::do_stuff() is not allowed to call Foo::quux() since Foo::quux() is using $this. Now the strange thing comes: instead of casting an error, PHP happily accepts the code. But the $this in Foo::quux is the same $this as in Bar::do_stuff(), i.e. $blabla, and that's why the output is 5. Is this behaviour intended? At least I couldn't find it documented anywhere. IMO the user should be warned when $this is used in a static function.

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 [2002-12-02 04:29 UTC] derick@php.net 
This is indeed intended behavior, but it should be documented. I have some logs about this, but can not find them now.

Derick
 [2002-12-02 05:17 UTC] krennmair at webdynamite dot com 
OK, but mark it in the documentation with a big red callsign. ;-)

But what I'm still interested in, is _why_ this is intended. The only reason I can think of is to confuse other programmers, but that would be absurd.
 [2002-12-03 21:06 UTC] wb at sapo dot pt 
i once had made the same 'mistake': i had split up a inheritance chain at some place but then i needed a function of the other inheritance trtee after the split. in fact, it worked out fine - so fine that i even didn?t notice that the superclass call was outside the inheritance chain. when i later noticed it i removed it because i got worried if it was desired behaviour or if it could disappear in some later version of php. but if you tell me that i can rely on it i will use it.
 [2004-08-18 09:34 UTC] vrana@php.net 
I think it's already documented at http://www.php.net/manual/en/keyword.paamayim-nekudotayim.php :

"In this context, there is a current object and it may have object variables. Thus, when used from WITHIN an object function, you may use $this and object variables."
 
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