Database update fejl

Hej Eksperter.
Jeg har siddet og prøvet at rette nogle fejl i 1 time nu, og hver gang jeg får 1 rettet opstår der en ny, selvom det er et ret simpelt update.
Koden ser sådan ud:

<?
session_start();
if(!$_SESSION[user_id] > 0) {
header("location:index.php");
exit;
}
require_once('connect.php');
$query = mysql_query("SELECT * FROM players WHERE id=$_GET[id]");
$user_data = mysql_fetch_assoc($query) or die(mysql_error());
$clanquery = mysql_query("SELECT * FROM clan WHERE clanleader=$_SESSION[user_id]");
$clan_data = mysql_fetch_assoc($clanquery) or die(mysql_error());
if($_SESSION[user_id] == $clan_data[clanleader])
{
if($clan_data[maxmembers] > $clan_data[clanmembers]) {
$sql = mysql_query("UPDATE players SET clanid = $clan_data[clanid] WHERE id=$_GET[id]") or die(mysql_error());
$name = $clan_data[clanname];
$sql = mysql_query("UPDATE players SET clanname = '$name' WHERE id=$_GET[id]") or die(mysql_error());
$sql = mysql_query("UPDATE players SET clanrank = 'Clan Member' WHERE id=$_GET[id]") or die(mysql_error());
$sql = mysql_query("UPDATE players SET clanapply = 0 WHERE id=$_GET[id]") or die(mysql_error());

$sql = mysql_query("UPDATE clan SET clanmembers = $clan_data[clanmembers] + 1 WHERE $clanid=clan_data[clanid]") or die(mysql_error());
echo $user_data[accountname] . " successfully added.";
header("location:clan.php");
}
else {
echo "Sorry there are too many members in your clan.";
}
}
?>

Lige nu har jeg den her fejl:
"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 '=clan_data[clanid]' at line 1".