PHP Warning: Call-time pass-by-reference has been deprecated

PHP Warning: Call-time pass-by-reference has been deprecated - argument passed by value; If you would like to pass it by reference, modify the declaration of file_set_status(). If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. However, future versions may not support this any longer. In sites/all/modules/subscriptions/subscriptions_mail.module on line 463.

This warning appears on several lines in the following modules:
subscriptions_mail: 463
subscriptions_mail: 467
subscriptions_content: 965

it is solved by taking function calls which pass a reference such as:
a($b, &$c)
and taking out the reference so that it is:
a($b, $c)
The reference is determined in the function declaration, so it is not needed here and causes php warnings.