phpqrcode is an external library that hasn't been updated in 2-3 years. On line 2940 of externals/phpqrcode/phpqrcode.php, it simply says $ret; which I believe is a bug in phpqrcode.
This task is to look into either updating phpqrcode from upstream (if there's a newer rev available), and to attach the quick work around which is $ret = null;.