Please help me see what's wrong

Please help me see what's wrong

if (!isset($_SESSION["sn"])){$_SESSION['sn']=0;setcookie('sn',$sn,time()+1);    if($_POST["submit"]){if($sn=3){

Alert ("Sorry, you have already clicked 3 times"); location. href = "zp. php ";

exit();}elseif ($sn=="tull"){

alert("123");location.href="zp.php";

exit();}$newitem=$_POST["zp_user"];$sql="insert into vote (awards,sn,pnone) values (1,'$sn',1)";if(mysql_query($sql)){    $sn==$_SESSION['sn']+1;}}if($_POST["submit2"]){if(empty($_POST['sn']))$_SESSION['zp_user']=$_POST['pnone'];$sql="insert into zp_user values('".($_POST['sn'])."')";if(mysql_query($sql)){}}}

Reply to discussion (solution)

If (mysql_query ($ SQL ))
{
$ Sn = $ _ SESSION ['SN '] + 1;

}
Why =?

How can code be separated ......

If ($ _ POST ["submit"]) {
If ($ sn = 3)
Should be =

If ($ _ POST ["submit"]) {
If ($ sn = 3) // =
{

= Yes value assignment
= If condition judgment is used