Actually if you use sendmail, there is NO NEED to use PHP at all. What I would do is setup procmail to find these emails and shoot them off to a perl script to parse out the text you need. From there you can insert the information into a MySQL database and write a PHP script that simply displays the information.
Oh and by the way, if they go to a specific email address, you can simply setup an alias (/etc/alias) to hand the message off to the script (again perl is great for this but I suppose you could use PHP CLI) . The input to catch is simply standard input (which pretty much all scripting languages can grab.) This method may be a little easier than procmail (procmail has a bit of a learning curve but ultimately is simple) but procmail is more configurable for email purposes (for example you can say parse and upload but deliver to box at the same time with procmail.)
I found an example of how somebody intercepted email based on content and sent it to a script. This is simply to give you an idea of how to do this. Of course you'll need to read up on procmail for installing and using it.
http://impressive.net/people/gerald/1997/11/ups-tracker/
