Question regarding the practical

Language: PHP
<?PHP
session_start();
if (!(isset($_SESSION[';login';]) && $_SESSION[';login';] != ';';)) {
	header ("Location: index.php");
}
?>

Language: PHP
<?php
 
$uname = "";
$pword = "";
$errorMessage = "";
$num_rows = 0;
 
function quote_smart($value, $handle) {
 
   if (get_magic_quotes_gpc()) {
       $value = stripslashes($value);
   }
 
   if (!is_numeric($value)) {
       $value = "';" . mysql_real_escape_string($value, $handle) . "';";
   }
   return $value;
}
 
if ($_SERVER[';REQUEST_METHOD';] == ';POST';){
	$uname = $_POST[';username';];
	$pword = $_POST[';password';];
 
	$uname = htmlspecialchars($uname);
	$pword = htmlspecialchars($pword);
 
 
	$user_name = "root";
	$pass_word = "";
	$database = "registration";
	$server = "localhost";
 
	$db_handle = mysql_connect($server, $user_name, $pass_word);
	$db_found = mysql_select_db($database, $db_handle);
 
	if ($db_found) {
 
		$uname = quote_smart($uname, $db_handle);
		$pword = quote_smart($pword, $db_handle);
 
		$SQL = "SELECT * FROM login WHERE uname = ';$uname'; AND pword = ';$pword';";
		$result = mysql_query($SQL);
		$num_rows = mysql_num_rows($result);
 
 
		if ($result) {
			if ($num_rows > 0) {
				session_start();
				$_SESSION[';login';] = "1";
				header ("Location: loggedin.php");
			}
			else {
 
				session_start();
				$_SESSION[';login';] = "";
				header ("Location: signup.php");
			}	
		}
		else {
			$errorMessage = "Error logging on";
		}
 
	mysql_close($db_handle);
	}
 
	else {
		$errorMessage = "Error logging on";
	}
}
 
?>

Language: PHP
<?PHP
session_start();
if (!(isset($_SESSION[';login';]) && $_SESSION[';login';] !== "")) {
	header ("Location: index.php");
}
?>

Language: PHP
if (!isset($_SESSION[';login';])) {