For instance I have a php function
<?php
function SayHi($msg){
return $msg;
}
?>
The following part triggers the onclick event
<a href="" onclick="">Click</a>
How can I merge the two such that when I click on Click, the php function is invoked..?
You can not. PHP is executed serverside, while javascript is executed clientside.
THe generic solution for this is to use asynchronous Javascript (ajax) to call on your webserver, which in turn can invoke PHP code to do various things.
jQuery is a very nice library to handle this, and much more.
There are many ways to do this, most are complicated.
One easier way is:
<?php
function myFunc() { die('yay'); }
if (isset($_REQUEST['myButton'])) myFunc();
?>
<form><button name="myButton">Click Me</button></form>
You'll want to use AJAX. You need to call a server-side resource from the JavaScript code, and jQuery has a very handy function for making the call to the server. But making the call from the JavaScript is only half the story, you'll also need something on the server listening for that call. It would essentially be another PHP script which acts as a page in and of itself, but would return data in the form of (most likely) JSON instead of HTML. It's not meant to be human-readable, but rather to be a sort of web service for your JavaScript code to use.
You can find a simple example here.
<script>tags to achieve that.