php - Error with my SQL syntax? Can't seem to find it

I might be going crazy, but I swear the code below was
working yesterday and now it's not working. I noticed it wasn't inserting into database
and I'm getting the error:

You
            have an error in your SQL syntax; check the manual that corresponds 
 to your
            MariaDB server version for the right syntax to use near 's',
 '39866',
            '9780448438047', 'Miss Spider's Sunny Patch Friends: Fun with Bounc' at line
            1

Here is the query
below. I know I'm not using escape strings/prepared statements, but that is because this
is just a localhost number
cruncher.

$insert =
            mysqli_query($connect, "INSERT INTO books 

 (subject, booknum,
            isbn, title, author, publisher, year, 
 format, mark, hurt, pubprice, srp,
            avail) VALUES
 ('$subject', '$booknumber', '$isbn', '$title', '$author',
            
 '$publisher', '$year', '$format', '$mark', '$hurt', 
 '$pubprice',
            '$srp', '$avail')") or
            die(mysqli_error($connect));

It
seems like even commenting out that line and just
doing:

$insert1 =
            mysqli_query($connect, "INSERT INTO books (subject) VALUES


            ('$subject')") or
            die(mysqli_error($connect));

Also
leads to an error. I'm guessing this means the error is actually located in my db
connection itself? I'm
using:

$connect
            = mysqli_connect("localhost", "root", "",
            "wholesale");
if(mysqli_connect_errno()) {
 echo "Failed to connect.
            Error: " .
            mysqli_connect_error();
}

Which
is included at the top of the
page:

include
            'db.php';

But like I
said, this application was working last night. All that has changed since then is I
turned off and on my computer and restarted xampp, as far as I can
remember.