Trying to automate something and understand globals

automate_something.php

<?php

$worked                =   false;
$GLOBALS['from']       =   "My devices <devices@myhouse.com>";
$GLOBALS['toEmail']    =   "me <me@address.com>";
$GLOBALS['subject']    =   "Home Automation Message";
$validDeviceIPs        =   ['192.168.0.110','192.168.1.109'];

/**
 * @param $device
 * @param string $action
 * @param string $additionalMessage
 * @return bool
 */

function doAction($device, $action = "OFF" ,$additionalMessage ="")
{
    $ok = false; //set a default response that assumes we couldn't command a device.
    switch(strtolower($action)){
        case "on":
            // ... whatever routine does that work. Assume it returns a bool $ok
            break;
        case "off":
            // ... turn it off, return bool $ok;
            break;
        default:
            $ok = false; // do nothing, retain the false and move on.
            break;
    }

    if($ok) {
        // appliance did whatever we asked, returned ok.
        // setup success email.
        $fullMessage = "Hi, \r\n I asked $device to turn $action and it did. Make sure the house isn't on fire.
                        \r\n Also, we were told to tell you $additionalMessage";
    } else {
        //error email
        error_log($device." failed to turn ".$action );
        $fullMessage = "YO!\r\n I asked $device to turn $action and it didn't or so we are told... You know how electronics are.
                        \r\n You may want to check it.\r\n Also, we were told to tell you $additionalMessage";
    }

    $mail = mail($GLOBALS['toEmail'], $GLOBALS['subject'].$message ,$fullMessage);
    if($mail){
        return true;
    } else {
        error_log("Failed to send a mail message about the $device turning $action");
        return false;
    }
    // failing all of that...
    return false;
}


if(isset($_POST['thing']) && isset($_POST['action']) && in_array($_POST['device'],$validDeviceIPs,true)){
    if("ON" == $_POST['action'] || "OFF" ==$_POST['action'] ){
        $worked = doAction($_POST['device'],$_POST['action'],"You can add an additional message in this paramater. It will be appended to the email");
        if($worked){
            $message = "Ok, I did the thing you wanted";
        } else {
            $message = "Something broke";
        }
    }
    echo $message;
}