Error message in PHP upgrade: Only variable references should be returned by reference

 When I update PHP5.3 to PHP7, the following error message shows:

"A PHP Error was encountered
Severity: Notice
Message: Only variable references should be returned by reference
Filename: core/Common.php"


CodeIgniter system/core/Common.php
My code location:
/var/www/html/new6sp/system/core/Common.php
line 257 change
        return $_config[0] =& $config;
to      
        $_config[0] =& $config;
        return $_config[0];