Php is parsed and executed on the web server at the time the page is requested. onclick events occur in the browser. You cannot "call" php functions from the browser. The browser can only make http requests to web servers (i.e. request pages through their URL.)
If you are learning PHP, developing PHP code, or debugging PHP code, do yourself a favor and check your web server log for errors and/or turn on full PHP error reporting in php.ini or in a .htaccess file to get PHP to help you.