Detecting exploits in web applications and how to proceed

What ways are there for detecting exploits in PHP/MySQL web applications (checking for certain characters or pieces of codes in the GET, POST, COOKIE arrays / using a library with a database that has all the patterns for common exploits, if any exist?) and how should I proceed when one is detected?

For example, if someone tried to find a SQL injection in my PHP/MySQL web application using the GET request method, should I store the action performed by the user in the database, have the application send me an e-mail, IP ban the user and display him/her a message "Sorry, but we have detected a harmful action from your account that will be reviewed. Your account has been disabled and certain features may be disabled from your IP address. If this is a mistake, please e-mail us with all the details."

Thanks.

Solution

Three things come to mind:

defensive coding, sanitize all input, prepare sql statements and use Suhosin
increase security of your site by breaking into it with a vulnerability scanner
log hacking attemtps with an Intrusion Detection System

If you feel a full fledged IDS is too much, try PHP IDS, as it does pretty much what you are asking for out of the box. Note that detecting intrusions at the PHP level might already be too late though to prevent an attack.

In case of a successful intrusion, I guess your best bet is taking the server offline and see what damage was done. You might have to consider hiring someone to do a forensic analysis of the machine in case you need to collect legally usable evidence.

If you feel you need to react to unsuccessful intrusion attempts and got the malicious user's IP, find out the ISP and inform him with as much details of the intrusion attempt as possible. Most ISPs have an abuse contact for these cases.