Questions about duplicate username detection

I want to solve the problem of duplicate user name Detection. Then, click a button to check whether the user name is valid for registration. In addition, does the php code obtain html form data after the user clicks the submit button ?, One can be verified on the client, and one can be serious on the server! & Lt ;! -- Functiongo () {varv username duplicate detection problem
I want to do the kind of user name, and then click a button to check whether the user name is valid for registration.
In addition, does the php code obtain html form data after the user clicks the submit button?

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One can be verified on the client, and one can be serious on the server!

Script
It's hard to see if you want it on the server.