execute php on click button

[civicf]

Hi,

I try write my first plugin for vbulletin. Could you explain me how I can execute my php when user click register button. In html code I see <form id="registerform" action="register.php...>, but I have no idea how execute my script. Some plugin uses ajax. Is it good solution?

edit:
I think I found solution. In plugin I will use hookname variable register_addmeber_complete and add my php