Your question is a bit confusing, as you refer to a text-based mail client and a PHP application and then specify that "the application" (i.e. your PHP application) to store email (or data extracted from that) in a database.
You can and probably should separate your sending application from your response processing application. The response processing can be done by calling any script from procmail, for that you need a .procmailrc file in the home directory of the receiving user that has the following:
You can leave out the Subject line if you want all mails to that email address processed, or use different selection criteria.
your_script I don't know of any commandline mail clients that directly put your material in a database. Since you probably should test the response anyway (if not to extract some extra database fields, at least to throw away spam) you might want to write
your_script in PHP using PECL to parse its content and store it in your database.
(Of course you can use other languages you are familiar with for this purpose as well).
If you are using postfix to receive emails on the machine this script runs on, make sure to call
procmail in /etc/postfix/main.cf:
mailbox_command = procmail -a "$EXTENSION"