PHP Error "Undefined index" -

hello i'm trying login system working on site , error saying notice: undefined index: user_login in c:\wamp\www\justbetweenfriends\home.php on line 4.

the database works , everthing else works besides error i'm getting now.

home.php code

 <?php error_reporting(e_all ^ e_deprecated); ?>     <?php     include ( "./inc/header.inc.php" );     echo $_session["user_login"];     ?> 

full code index.php

<?php error_reporting(e_all ^ e_deprecated); ?> <?php include( "./inc/header.inc.php" ); ?> <?php  $reg = @$_post['reg']; //declaring variables prevent errors $fn = ""; //first name $ln = ""; //last name $un = ""; //username $em = ""; //email $em2 = ""; //email 2 $pswd = ""; //passoword $pswd2 = ""; // password 2 $d = ""; // sign data $u_check = ""; // check if username extsts //registration form $fn = strip_tags(@$_post['fname']); $ln = strip_tags(@$_post['lname']); $un = strip_tags(@$_post['username']); $em = strip_tags(@$_post['email']); $em2 = strip_tags(@$_post['email2']); $pswd = strip_tags(@$_post['password']); $pswd2 = strip_tags(@$_post['password2']); $d = date("y-m-d"); // year - month - day  if ($reg) { if ($em==$em2) { // check if user exists $u_check = mysql_query("select username users username='$un'"); // count amount of rows username = $un $check = mysql_num_rows($u_check); if ($check == 0) { //check of fields have been filed in if ($fn&&$ln&&$un&&$em&&$em2&&$pswd&&$pswd2) { // check passwords match if ($pswd==$pswd2) { // check maximum length of username/first name/last name not exceed 25 charcters if (strlen($un)>25||strlen($fn)>25||strlen($ln)>25) { echo "the maximum limit username/first name/last name 25 characters!"; } else { // check maximum length of password not exceed 25 characters , not less 5 characters if (strlen($pswd)>30||strlen($pswd2)<5) { echo "your password must between 5 , 30 characters long!"; } else { //encrypt password , password 2 using md5 before sending database $pswd = md5($pswd); $pswd2 = md5($pswd2); $query = mysql_query("insert users values ('','$un','$fn','$ln','$em','$pswd','$d','0')"); die("<h2>welcome justbetweenfriends</h2>login account started ..."); } } } else { echo "your passwords don't match!"; } } else { echo "please fill in fields"; } } else { echo "username taken ..."; } } else { echo "your e-mails don't match"; } }  // user login code  if (isset($_post["user_login"]) && isset($_post["password_login"])) { $user_login = preg_replace('#[^a-za-z0-9]#i', '', $_post["user_login"]); $password_login = preg_replace('#[^a-za-z0-9]#i', '', $_post["password_login"]); $password_login_md5 = md5($password_login); $sql = mysql_query("select id users username='$user_login' , password='$password_login_md5' limit 1"); //check thir existance $usercount = mysql_num_rows($sql); //count number of rows returned if ($usercount == 1) { while($row = mysql_fetch_array($sql)){ $id = $row["id"]; } $_session["user_login"] = $user_login; header("location: index.php"); exit(); }else{     echo " information incorrect, try again"; exit(); } } ?> <div style="width: 800px; margin: 0px auto 0px auto;"> <table> <tr> <td width="60%" valign="top"> <h2>already member? sign in below!</h2> <form action="index.php" method="post"> <input type="text" name="user_login" size="25" placeholder="username"><br /><br /> <input type="text" name="password_login" size="25" placeholder="password"><br /><br /> <input type="submit" name="login" value="login"> </form> </td> <td width="40%" valign="top"> <h2>sign below!</h2> <form action="index.php" method="post"> <input type="text" name="fname" size="25" placeholder="first name"><br /><br /> <input type="text" name="lname" size="25" placeholder="last name"><br /><br /> <input type="text" name="username" size="25" placeholder="username"><br /><br /> <input type="text" name="email" size="25" placeholder="email address"><br /><br /> <input type="text" name="email2" size="25" placeholder="email address (again)"><br /><br /> <input type="text" name="password" size="25" placeholder="password"><br /><br /> <input type="text" name="password2" size="25" placeholder="password (again)"><br /><br /> <input type="submit" name="reg" value="sign up!"> </form> </td> </tr> </table> <?php include( "./inc/footer.inc.php" ); ?> 

it's not error persay, more of warning php. php allows call variable if hasn't been set, warn if it, security reasons. more detailed explanation can found here.