I don't know anything about mac either, but the behaviour you are experiencing is the same as an unknown or invalid php handler for the webserver, so it doesn't know what to do with a .php file. I'd go over your logs to see if you've experienced an event causing the php module to fail (which is likely a part of the error.log for apache itself), which could explain why its attempting to download the file instead of parsing it as expected.
Since I'd expect this is a development machine, I wouldn't waste too much time concerning yourself of this unless it was caused by something you have written. I'd just reboot the machine and see if that fixes the problem. Running slow says something is wrong, whether it be hardware or software related.