Receivinig an email into a web site

I would like to send someone an email when they apply for an account on my website and then have them respond to it from their email. Once they respond, how do I get the information I need from the email to be able to process it with PHP and MySQL? Thanks

After more exploring, I think what I am looking for is the PHP mail() function. I am playing with that now and it seems to be doing what I want so at least for now, I will consider this closed. Thanks for the input.

Quite a vague question. When sending the email, can you not just give them the url to a form which they fill in with the relevant info. Then you process the form and record the info into your database.