Consult the implementation principle of verification code

Please tell me the implementation principle of verification code
Verification code is very necessary, but why like Baidu Bar those, its verification code above to get some spots to interfere with visual identification? I think it's OK not to disturb.

And what is the principle of the implementation and application of the verification code? Can someone please give a brief explanation? Thank you

My feeling is to use the verification code should be used in the HTML of the hidden element, do not know the right?

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Without disturbing it, it is easy for others to use orc recognition technology to disguise your verification code so they can easily make information release robots.

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Fill in the interface to show the random number of users to fill out the form, and this random number written to the session, the post-processing code to determine whether the user has this commit random number and compared with the value in the session, and then ....
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But how does this server side know $_session[' verify ' this SESSION variable belongs to which client browser?

A: Each browsing client will establish a unique session with the server
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