Only the operating system has ultimate control over processes. PHP has no power to intervene in MySQL processes unless it asks the operating system to do it (via a script) or MySQL allows it to, which it does.
MySQL provides SQL queries to view and kill processes. You can run these queries using PHP. This has it's limits as your database server could be choking, where running these queries may have no effect as they may never reach the database. At that point executing a script on the system may be an alternative option.
Have a look at the KILL syntax.
“The best thing about a boolean is even if you are wrong, you are only off by a bit.”