For a popup javascript is required. Put this in your header:
<script>
function myFunction()
{
alert("I am an alert box!"); // this is the message in ""
}
</script>
And this in your body:
<input type="button" onclick="myFunction()" value="Show alert box">
When the button is pressed a box pops up with the message set in the header.
This can be put in any html or php file without the php tags.
-----EDIT-----
To display it using php try this:
<?php echo '<script>myfunction()</script>'; ?>
It may not be 100% correct but the principle is the same.
To display different messages you can either create lots of functions or you can pass a variable in to the function when you call it.