PHP]Secure page password protection

<?php
echo "<body bgcolor=black>";

session_start(); //starts the session. you will get a PHPSESSID cookie

$admin_user_name = "root"; //Change this with the username you want
$admin_password = "toor"; //Change this with the password you want

if (!isset($HTTP_SESSION_VARS['user'])) { //If user is not blank

    if(isset($HTTP_POST_VARS['u_name']))  //If user is not blank

        $u_name = $HTTP_POST_VARS['u_name']; //Set $u_name


    if(isset($HTTP_POST_VARS['u_password']))  //If password is not blank

        $u_password = $HTTP_POST_VARS['u_password']; // Set Password


    if(!isset($u_name)) {

        ?>

        <HTML>

        <HEAD>

        <TITLE><?php echo $HTTP_SERVER_VARS['HTTP_HOST']; ?> : Authentication Required</TITLE>

        </HEAD>

        <BODY bgcolor=#ffffff>

        <table border=0 cellspacing=0 cellpadding=0 width=100%>

             <TR><TD>

             <font face=verdana size=2><B> </b> </font></td>

             </tr></table>

        <P></P>

        <font face=verdana size=2>

        <center>

        <?php

        $form_to = "http://$HTTP_SERVER_VARS[HTTP_HOST]$HTTP_SERVER_VARS[PHP_SELF]";


        if(isset($HTTP_SERVER_VARS["QUERY_STRING"]))

        $form_to = $form_to ."?". $HTTP_SERVER_VARS["QUERY_STRING"];


        ?>

        <form method=post action=<?php echo $form_to; ?>>

        <table border=0 width=350>

        <TR>

        <TD><font face=verdana size=2><B><font color=white>User Name</font></B></font></TD>

        <TD><font face=verdana size=2><input type=text name=u_name size=20></font></TD></TR>

        <TR>

        <TD><font face=verdana size=2><B><font color=white>Password</font></B></font></TD>

        <TD><font face=verdana size=2><input type=password name=u_password size=20></font></TD>

        </TR>

        </table>

        <input type=submit value=Login></form>

        </center>

        </font>

        </BODY>

        </HTML>

        <?php

        exit;

    }

    else {


        function login_error($host,$php_self) { //function for returning error page

            echo "<HTML><HEAD>

            <TITLE>$host :  Login</TITLE>

            </HEAD><BODY bgcolor=white>

            <table border=0 cellspacing=0 cellpadding=0 width=100%>

                 <TR><TD align=left>

                 <font color=white><font face=verdana size=2><B>  You Need to login to view this page. </b> </font></font></td>

                 </tr></table>

            <P></P>

            <font face=verdana size=2>

            <center>";


            echo "Error: You are not Logged in!

            <font color=white><B><a href=$php_self>Click here</a></b> to login again.<P></font>

            </center>

            </font>

            </BODY>

            </HTML>";

            session_unregister("adb_password"); //Unregister's the password you entered from the session since its invalid

            session_unregister("user"); //Unregister's the username you entered from the session since its invalid

            exit;

        }


        $user_checked_passed = false;


        if(isset($HTTP_SESSION_VARS['adb_password'])) { //if password is not blank


            $adb_session_password = $HTTP_SESSION_VARS['adb_password'];

            $adb_session_user = $HTTP_SESSION_VARS['user'];


            if($admin_password != $adb_session_password) //if entered password not equal to the page password then

                login_error($HTTP_SERVER_VARS['HTTP_HOST'],$HTTP_SERVER_VARS['PHP_SELF']); //calls the login_error function

            else {

                $user_checked_passed = true;

            }

        }


        if($user_checked_passed == false) {


            if(strlen($u_name)< 2)

                login_error($HTTP_SERVER_VARS['HTTP_HOST'],$HTTP_SERVER_VARS['PHP_SELF']);


            if(isset($admin_password)) {


                if($admin_password == $u_password) {


                    session_register("adb_password");

                    session_register("user");


                    $adb_password = $admin_password;

                    $user = $u_name;

                }

                else {

                    login_error($HTTP_SERVER_VARS['HTTP_HOST'],$HTTP_SERVER_VARS['PHP_SELF']);

                                //logs ip to iplog.txt for more security of who wanted to access the page
                                $fp = fopen('IPLog.txt', 'a');
                                fwrite($fp, $_SERVER['REMOTE_ADDR']." Accessed at ".date("j F, Y, g:i a")."\n");
                                fclose($fp);

                }

            }

            else {

                login_error($HTTP_SERVER_VARS['HTTP_HOST'],$HTTP_SERVER_VARS['PHP_SELF']);

                                //logs ip to iplog.txt for more security of who wanted to access the page
                                $fp = fopen('IPLog.txt', 'a');
                                fwrite($fp, $_SERVER['REMOTE_ADDR']." Accessed at ".date("j F, Y, g:i a")."\n");
                                fclose($fp);
            }


            $page_location = $HTTP_SERVER_VARS['PHP_SELF'];

            if(isset($HTTP_SERVER_VARS["QUERY_STRING"]))

            $page_location = $page_location ."?". $HTTP_SERVER_VARS["QUERY_STRING"];

            header ("Location: ". $page_location);

        }
    }
}

?>