How to pop an alert message box using PHP?

You could use Javascript:

// This is in the PHP file and sends a Javascript alert to the client
$message = "wrong answer";
echo "<script type='text/javascript'>alert('$message');</script>";

Create function for alert

<?php
alert("Hello World");

function alert($msg) {
    echo "<script type='text/javascript'>alert('$msg');</script>";
}
?>