mysql_connect is part of the mysql library that has been deprecated in PHP7 in favor of the mysqli library. PhreeBooks 3.7 will not run on PHP7. You will need to run a simultaneous install of PHP5 along side of PHP7 to avoid this error. If you Google running PHP5.x AND php 7 on same server in ubuntu you should be able to find a how to to help get this done. I believe PhreeBooks R4.0 in development fixes this issue, it’s release is due out mid year, see the GitHub development site for current information.