You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
The query works but the error report "bugs" me :)
//the vars i use
$pos = $_POST['positie']; //this can be B1 till M100
$kolom = $pos[0]; //get first char of the $pos string
$rij = substr($pos, 1, 3); //get the rest of the chars
$sql= mysql_query("UPDATE floorplan SET available='0' WHERE kolom='$kolom' AND rij='$rij'") or die( mysql_error() );
the kolom is varchar(4), rij is int(4) and avaiable is a BOOLEAN.
In mysql, boolean is simply synonym for TINYINT(1). A value of 0 is considered FALSE. So, change
$rij = substr($pos, 1, 3);
to
$rij = intval(substr($pos, 1, 3));
and query to
"UPDATE floorplan SET available=0 WHERE kolom='$kolom' AND rij=$rij"
would solve your problem but like others said, you should think about SQL Injection.
Suppose $pos has a value of "B1".
Then, substr($pos, 1, 3); will return an out of bounds exception.
Try with substr($pos, 1);