Novice question: How to change the contents of get, the page does not change

Novice question: How to change the contents of get, the page did not change AH

 
 Require ' inc/function.php ';
Header ("Expires:-1");
Header ("Cache-control:no-cache");
Header ("Pragma:no-cache");

$user =$_post["User"];
if (Isset ($user)) {
Echo ("0| |");
}
else{
$con = mysql_connect (DBHOST,DBUSER,DBPW);
if (! $con)
  {
  Die (' Database connection error: '. mysql_error ());
  }

mysql_select_db (DBNAME, $con);

$result = mysql_query ("Select Uid,password,salt from ' pre_ucenter_members ' where username= '". Checkstr (user). "'");
Mysql_close ($con);

$row =mysql_fetch_row ($result);
Echo ("$row [0]| $row [1]| $row [2]");
}
?>

This page executes once, executes again, modifies the parameter after the user=, the execution result does not have any change. $user =$_post["User"];
Switch
$user =$_get["User"];

Didn't you use post to receive it? Change? user= back parameters, no effect on post

