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Complete, simple working example of login screen and check on a unique

Complete, simple working example of login screen and check on a unique
page using php functions cookies and mysql database.

<?php 
mysql_connect("localhost","DBUser") or die ("Error connecting with the server. Try again later. 
Thanks."); 
$db="DBClients"; 

if (!$mysql_login)   { 
        // check if $username and $password are passed through as query string 
    if($username && $password) { 
        include("./dbConn.php3");   
            // If you want to have the passwords in the databases crypted. 
        $password = md5($password);   

        $loc="Location: http://$SERVER_NAME$SCRIPT_NAME";. 
        $sql = "select Password from users_t where Username='$username'"; 
        $r = mysql_db_query($db,$sql); 

            //check if the result of the query has at least one row.Or it will redirect the page. 
        if(!mysql_num_rows($r)) header($loc);   

        $user = mysql_fetch_array($r); 
        if($user["Password"] == $password) { 
                //note the space between the username and the passoword. It will be 
                //used below to slice the cookie. 
            setcookie("mysql_login","$username $password");   
        $msg = "<meta http-equiv=\"Refresh\" content=\"0;url=$SCRIPT_NAME\">"; 
        //The use of the http header is inconsistent with many browser if you have 
                //already setted a cookie. So, after the setcookie command the script uses 
                //the meta tag to obtain the same result. 
        }else{ 
        header($loc); 
        } 
    }else{ 
?> 

        <html> 
        <title>Login</title> 
        <body bgcolor="yellow" text="black"> 
        <form method="post" action="<?php echo($SCRIPT_NAME)?>"> 
        <center><font size=+5><b>Welcome!</b></font></center> 
        <br><br><br> 
        <table cellspacing=0 cellpadding=0 width=320 align="center"> 
        <tr><td>Username:</td><td> 
        <tr><td><input name="username" type="text" width=10></td></tr> 
        <tr><td>Password: </td><td> 
        <tr><td><input name="password" type="password" width=10></td></tr> 
        <tr><td colspan=2 align="center"><input name="login" type="submit"></td></tr> 
                </form>         
                </table> 
        <body> 
        </html> 

        <?php 
        die(); //This will prevent PHP from showing actual page. See above. 
    } 
}else{ 
    //include("./dbConn.php3");   
    $chk=0; 
        // Set an array called $user with the contents of the cookie 
        //sliced at the space between username and password, see above. 
    $user = explode(" ","$mysql_login");   
    $sql = "select Password from users_t where username='$user[0]'";   
    $r = mysql_db_query($db,$sql); 

        //check if the result of the query has at least one row.Or it set the $chk variable. 
    if(!mysql_num_rows($r)) $chk=1;   
    $chkusr = mysql_fetch_array($r); 
    if ($user[1]!=$chkusr["Password"]) $chk=1; 
    if ($chk) { 
            // To reset the cookie, the setcookie function is used here without argoument.   
        setcookie("mysql_login"); 
            // see above. 
        echo("<meta http-equiv=\"Refresh\" content=\"0;url=$SCRIPT_NAME\">"); 
    } 

} 

//After here the actual page starts and only registered user can see it. 
?>