Warning: Use of undefined constant php - assumed 'php' (this will throw an Error in a future version of PHP) in * on line 1

I'm following the tutorial here to create a login page using MySQL. The code seemed to be working flawlessly when I left my computer 2 days ago. Now I come back to work on it, haven't changed anything, and I'm greeted with this warning:

Warning: Use of undefined constant php - assumed 'php' (this will
throw an Error in a future version of PHP) in /filelocation/login.php
on line 1

Line 1 in login.php literally says only this:

It also seems like this warning is breaking the form. When I hit the "login" button, nothing happens besides being redirected right back to the login page to try again.

Why would this be happening?

edit* As requested here is the fist couple lines of my code. Simply copy-pasted:

// Initialize the session
session_start();

// Check if the user is already logged in, if yes then redirect him to welcome page
if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){
    header("location: welcome.php");
    exit;
}