PHP Insert Query Problems

[Alistair Beasley]

$tSession = session_id();
$query = "INSERT INTO usersession (SessionID) VALUES ('$tSession')";
$result = mysql_query($query) or die ("Couldn't Execute Query 1");

//from index down to } is a while loop that generates 15 unique random numbers, these numbers will then be stored in the database.
$index[0] = "0";

for ($n=1; $n < 16; $n++)
{
$rand = mt_rand(1, 200);
While( array_search($rand, $index) )
$rand = mt_rand(1, 200);
$query = "UPDATE usersession SET $n='$index[$n]' WHERE SessionID='$tSession'";
$result = mysql_query($query) or die ("Couldn't Execute Query 2");
$index[$n] = $rand;

echo "$n: $index[$n]<br>";
}

Ok so I am creating a quiz that will need to ask questions at random while ensuring the random numbers aren't duplicating, this works fine, the probelm I am having is with the UPDATE query.

I have 15 columns lablled 1, 2, 3, 4, 5... 15. So what I want to do is insert the randomly generated unique number into the table, $n='$index[$n]' is where the coding is falling down.

What am I doing wrong? I'm sure its just a simple error involving " or ' missing but I can't find any info on it.