<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
if(isset($_POST['submit']))
{
    define('GENERAL_PRICE', 5.00);
    define('VIP_PRICE', 20.00);

    if(isset($_POST['general']) && isset($_POST['vip']))
    {
        $general = $_POST['general'];
        $vip = $_POST['vip'];

        //set discount
        if($general + $vip >= 5)
        {
            $discount = 0.75;
        } else {
            $discount = 0;
        }

        // calculate cost
        $cost = ((GENERAL_PRICE  * $discount) * $general) + ((VIP_PRICE * $discount) * $vip);
        echo $cost;

    } else {
        $error = 'You must fill out the number of general and VIP tickets';
        echo $error;
    }
}
?>

<form action="demo.php" method="post">
    <input type="text" name="general" placeholder="General">
    <br/><br/>
    <input type="text" name="vip" placeholder="VIP">
    <br/><br/>
    <button type="submit" name="submit">Calculate</button>
</form>