PHP user registration example code, suitable for beginners_PHP tutorial

php tutorial user registration example code, suitable for beginners
if($_POST[submit]){

checkusername($_POST[username]);

if($_SESSION["code"]==$_POST[code])

{

$username= str_replace(" ","",$_POST[username]);
$password=md5($_POST[password].ALL_PS);
$sql="INSERT INTO `member`.`user_list` (`uid`, `m_id`, `username`, `password`) VALUES (NULL, '$_POST[select]', '$username','$password ' )";
Mysql tutorial_query($sql);
echo "Registration successful";
}

else
{
echo "Verification code error"."
";
Session_destroy();
}
}

function checkusername($username)
              {
               $sql2 = "select uid from user_list where username='".mysql_escape_string($username)."'";
                 $result = mysql_query($sql2) or die ('ERROR: '.mysql_error());
If(mysql_num_rows($result)>0)
                                                  {
echo "The username already exists, please change it!";

exit();
                 }
            else{
echo "<script> location.href='reg.php'</script>";
                 }

           }

?>