How can you enhance the difficulty of brute force of PHP verification code??

How can you enhance the difficulty of brute force of PHP verification code??

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How can you enhance the difficulty of brute force of PHP verification code??

The simplest way is to give up on your own and access Google's reCAPTCHA

But I'm going to post a few websites to destroy the landlord wants to strengthen the verification code to prevent the robot/spam.

http://dama2.com/
http://www.sz789.net/

Conclusion: Give up, will only hurt their own user experience, there is a decent verification code to put a look on the line

Characters are connected together, and then moderately distorted deformation, look at Google and Yahoo's verification code is basically the case, now noise reduction technology is relatively high, too much noise also affect the user experience

What is the specific type of brute force that the landlord says?
If it is a constant attempt, so long as each verification fails to regenerate the verification code, in the case of too many attempts to increase the length of the verification code, complexity

Accidentally saw a relatively tall on the verification code .... http://geetest.com/

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