PHP is a server-side application so there, so you cannot directly call it onClick() because that event happens on the client side.
However, you can call it via JavaScript! More specifically aJax.
Here's a good place to start
Here's a quick example from What is Sleep`s answer
AJax:
function callPHP() {
$.ajax ({
url: "yourPageName.php",
data: { action : assign }, //optional
success: function( result ) {
//do something after you receive the result
}
}
PHP:
if ($_POST["action"] == "assign")
{
assign(your parameters); //You need to put the parameters you want to pass in
//the data field of the ajax call, and use $_POST[]
//to get them
}