Is it possible to call a php function by using a hyperlink onclick event?

The short answer is no.

PHP is a Server Side language and Javascript is commonly implemented as
a Client Side language.

The two usually cannot (and should not) meet.

That said, depending on what you want to do you could use AJAX to
better integrate your Javascript with your Server Side coding.

A good article I've found on the subject is here, however a google
search for AJAX will yield a wealth to explore.

http://www.phpbuilder.com/columns/kassemi20050606.php3