I found the following answer quite interesting:

http://stackoverflow.com/questions/8404775/how-to-identify-w...

"There are two general ways to detect robots and I would call them "Polite/Passive" and "Aggressive". Basically, you have to give your web site a psychological disorder.
Polite

These are ways to politely tell crawlers that they shouldn't crawl your site and to limit how often you are crawled. Politeness is ensured through robots.txt file in which you specify which bots, if any, should be allowed to crawl your website and how often your website can be crawled. This assumes that the robot you're dealing with is polite.
Aggressive

Another way to keep bots off your site is to get aggressive.

User Agent

Some aggressive behavior includes (as previously mentioned by other users) the filtering of user-agent strings. This is probably the simplest, but also the least reliable way to detect if it's a user or not. A lot of bots tend to spoof user agents and some do it for legitimate reasons (i.e. they only want to crawl mobile content), while others simply don't want to be identified as bots. Even worse, some bots spoof legitimate/polite bot agents, such as the user agents of google, microsoft, lycos and other crawlers which are generally considered polite. Relying on the user agent can be helpful, but not by itself.

There are more aggressive ways to deal with robots that spoof user agents AND don't abide by your robots.txt file:

Bot Trap

I like to think of this as a "Venus Fly Trap," and it basically punishes any bot that wants to play tricks with you.

A bot trap is probably the most effective way to find bots that don't adhere to your robots.txt file without actually impairing the usability of your website. Creating a bot trap ensures that only bots are captured and not real users. The basic way to do it is to setup a directory which you specifically mark as off limits in your robots.txt file, so any robot that is polite will not fall into the trap. The second thing you do is to place a "hidden" link from your website to the bot trap directory (this ensures that real users will never go there, since real users never click on invisible links). Finally, you ban any IP address that goes to the bot trap directory.

Here are some instructions on how to achieve this: Create a bot trap (or in your case: a PHP bot trap).

Note: of course, some bots are smart enough to read your robots.txt file, see all the directories which you've marked as "off limits" and STILL ignore your politeness settings (such as crawl rate and allowed bots). Those bots will probably not fall into your bot trap despite the fact that they are not polite.

Violent

I think this is actually too aggressive for the general audience (and general use), so if there are any kids under the age of 18, then please take them to another room!

You can make the bot trap "violent" by simply not specifying a robots.txt file. In this situation ANY BOT that crawls the hidden links will probably end up in the bot trap and you can ban all bots, period!

The reason this is not recommended is that you may actually want some bots to crawl your website (such as Google, Microsoft or other bots for site indexing). Allowing your website to be politely crawled by the bots from Google, Microsoft, Lycos, etc. will ensure that your site gets indexed and it shows up when people search for it on their favorite search engine.

Self Destructive

Yet another way to limits what bots can crawl on your website, is to serve CAPTCHAs or other challenges which a bot cannot solve. This comes at an expense of your users and I would think that anything which makes your website less usable (such as a CAPTCHA) is "self destructive." This, of course, will not actually block the bot from repeatedly trying to crawl your website, it will simply make your website very uninteresting to them. There are ways to "get around" the CAPTCHAs, but they're difficult to implement so I'm not going to delve into this too much.
Conclusion

For your purposes, probably the best way to deal with bots is to employ a combination of the above mentioned strategies:

Filter user agents.
Setup a bot trap (the violent one).

Catch all the bots that go into the violent bot trap and simply black-list their IPs (but don't block them). This way you will still get the "benefits" of being crawled by bots, but you will not have to pay to check the IP addresses that are black-listed due to going to your bot trap."