If you want to call a function on clicking of submit button then you have
to use ajax or jquery,if you want to call your php function after submission of form
you can do that as :
<html>
<body>
<form method="post" action="display()">
<input type="text" name="studentname">
<input type="submit" value="click">
</form>
<?php
function display()
{
echo "hello".$_POST["studentname"];
}
if($_SERVER['REQUEST_METHOD']=='POST')
{
display();
}
?>
</body>
</html>