为什么我不能设置会话变量？

Question

I had already used this code 2 times, but now doesn't work and i don't know why. Below is the code from the file process_login.php. The code works fine, but when i get to admin.php can´t echo $_SESSION['USER'] and the same isn´t set. Thanks in advance.

<?php
    session_start(); 
    mysql_connect('localhost','root','root');   
    mysql_select_db('user_db'); 

    $user=$_POST['user']; 
    $password=$_POST['password'];

    $sql="SELECT * FROM users WHERE username='".$user."' AND password='".$password."'";     
    $res=mysql_query($sql); 

    if (mysql_num_rows($res)>0) 
    {           
        $row=mysql_fetch_array($res); 
        $_SESSION['USER']=$row['username']; 
        header("location: admin.php");                              
    }   
    else
    {   
        header("location: login.php");
    }   

?>

Answer 1

Using mysql_fetch_array will return an array with id numbers while mysql_fetch_assoc will return textual representation of the columns.

//mysql_fetch_array()
$myarray = array(
    [0] => "BILLYBOB"
);

//mysql_fetch_assoc()
$myarray = array(
    ["USER"] => "BILLYBOB"
);