How can I kill a process by name instead of PID on Linux

Question

Sometimes when I try to start Firefox it says "a Firefox process is already running". So I have to do this:

jeremy@jeremy-desktop:~$ ps aux | grep firefox jeremy 7451 25.0 27.4 170536 65680 ? Sl 22:39 1:18 /usr/lib/firefox-3.0.1/firefox jeremy 7578 0.0 0.3 3004 768 pts/0 S+ 22:44 0:00 grep firefox jeremy@jeremy-desktop:~$ kill 7451

What I'd like is a command that would do all that for me. It would take an input string and grep for it (or whatever) in the list of processes, and would kill all the processes in the output:

jeremy@jeremy-desktop:~$ killbyname firefox

I tried doing it in PHP but exec('ps aux') seems to only show processes that have been executed with exec() in the PHP script itself.