Unknown Column In Where Clause mysql query - php

Question

My query is $query = "SELECT * FROM cartmatch WHERE CARTNO=$cart4"; and I'm receiving an error that says "Unknown column 'M833' in 'where clause'". Just so you know, cart4=M833.
::EDIT::
For some reason, nothing is showing. Here is the code on the page.
<?php
$cart1 = rawurldecode($_GET["path"]);
list( , , , , , $cart2) = explode ("\\", $cart1);
$cart3 = $cart2;
list($cart4) = explode (" ", $cart3);
$con = mysql_connect("SERVER","USERNAME","PASSWORD");
if (!$con)
{
die('Could not connect: ' . mysql_error());
}
mysql_select_db("cartmatch", $con);
$result = mysql_query("SELECT * FROM cartmatch WHERE CARTNO='$cart4'");
while($row = mysql_fetch_array($result))
{
echo '<form enctype="multipart/form-data" action="album.php" method="POST">Please enter press save.<br><br><input name="ID" type="hidden" value=';
echo $_GET["ID"];
echo ' ><input name="enabled" type="hidden" value=';
echo $_GET["enabled"];
echo ' ><input name="artist" type="hidden" value=';
echo $_GET["artist"];
echo ' ><input name="title" type="hidden" value="';
echo $_GET["title"];
echo '" >Name:<br/><input name="album" type="text" autofocus="autofocus" value="';
echo $row['ALBUM'];
echo '" ><input type="submit" name="edit" value="Save"></form>';
}
mysql_close($con);
?>

Answer 1

Change the query to:
"SELECT * FROM cartmatch WHERE CARTNO='$cart4'"
and change
list($cart4) = explode (" ", $cart3);
to
list($cart4) = explode ("+", $cart3);

Answer 2

Change the WHERE section to
CARTNO='$cart4'