Some background info: Has been observed once on given site. Last code update to master for core and extensions was about 15 minutes before the issue was logged (17-Apr-2013 20:27:45 UTC). The updated code was about 2 days worth.

Simple repro:

$ php maintenance/eval.php

$l=new StubUserLang;

$n='1';

$t=Title::newFromText($n);

$l->findVariantLink($n,$t);

PHP Warning: Parameter 1 to Language::findVariantLink() expected to be a reference, value given in ./includes/StubObject.php on line 79
PHP Stack trace:
PHP 1. {main}() ./maintenance/eval.php:0
PHP 2. eval() ./maintenance/eval.php:72
PHP 3. StubUserLang->findVariantLink() ./maintenance/eval.php(72) : eval()'d code:1
PHP 4. StubUserLang->__call() ./maintenance/eval.php(72) : eval()'d code:1
PHP 5. StubObject->_call() ./includes/StubObject.php:177
PHP 6. call_user_func_array() ./includes/StubObject.php:79

Warning: Parameter 1 to Language::findVariantLink() expected to be a reference, value given in ./includes/StubObject.php on line 79

Call Stack:

 0.0003     236024   1. {main}() ./maintenance/eval.php:0
18.7853    8924472   2. eval('$l->findVariantLink($n,$t);;') ./maintenance/eval.php:72
18.7853    8924672   3. StubUserLang->findVariantLink() ./maintenance/eval.php(72) : eval()'d code:1
18.7853    8925112   4. StubUserLang->__call() ./maintenance/eval.php(72) : eval()'d code:1
18.7853    8925144   5. StubObject->_call() ./includes/StubObject.php:177
18.9612   10705640   6. call_user_func_array() ./includes/StubObject.php:79

A direct cause is that stub objects can't handle calls with references. However:

1. $wgContLang is never initialized as a stub object
2. findVariantLink() is always expected to run on $wgContLang

Looks as if something is calling findVariantLink() on $wgLang or similar stuff incorrectly? A backtrace would be useful...

• Bug 44583 has been marked as a duplicate of this bug. ***

Closing this as fixed. Not observed in a long time. The scenario from comment 2 can still be reproduced.