Check Availability button

I have a form that accepts user info.
I want to create a button that enables the user to check for the availability of the username entered before continuing with the rest of the form.

I have tried using the button onclick to call a JS function. The JS function have codes to compare the username with database select.

Is my approach wrong?
Can the button onclick event call a PHP function?

Thanks.

Sure it could call php function. Onclick event just will redirect you to some php file where you could check for a user name. You could take user name from form with getElementById(js property).