我的全局变量在函数中无法识别（PHP）

My programming level is fairly elementary so a 'coding for idiots' type of explanation would be great...

I have the following code:

<?php

$host = 'localhost';
$user = 'user';
$password = 'password';
$db_name = 'db_name';
$connect = mysqli_connect($host, $user, $password);

mysqli_select_db($connect, $db_name) or die ("Couldn't connect"); 

function roll_die() {

    $throw = rand(1, 6);
    return $throw;
}

function get_subtotal() {

    $query = "SELECT * FROM throws";
    $result = mysqli_query($connect, $query);

    while ($row = $result->fetch_assoc()) {
        echo $row['value']."<br>";
    }
}

?>

I get an error because the '$connect' in the function subtotal() is apparently undefined. How can that be if it's defined at the top of the page? Wouldn't that make it a global function?

Please don't just give me the correct code to fix this. Could you explain what's going on and how PHP defines and stores variables?

Thanks!

Ok, I found the answer.

At the top of the page:

$connect = mysql_connect($host, $user, $password);

Then within the function:

global $connect;

Actually, I found the answer in another question which was essentially asking the same thing, so apologies for the repost.