Only variable references should be returned by reference - Codeigniter
Edit filename: core/Common.php, line number: 257
Before
return $_config[0] =& $config;
After
$_config[0] =& $config;
return $_config[0];
Update
Added by NikiC
In PHP assignment expressions always return the assigned value. So $_config[0] =& $config returns $config - but not the variable itself, but a copy of its value. And returning a reference to a temporary value wouldn't be particularly useful (changing it wouldn't do anything).
Update
This fix has been merged into CI 2.2.1 (https://github.com/bcit-ci/CodeIgniter/commit/69b02d0f0bc46e914bed1604cfbd9bf74286b2e3). It's better to upgrade rather than modifying core framework files.
this has been modified in codeigniter 2.2.1...usually not best practice to modify core files, I would always check for updates and 2.2.1 came out in Jan 2015