php - Variable errors in session for admin pages -

i creating login website. can code below working: lets me log in! yet can't start session work: people can still pages via url.

log in php:

<?php     //calling connection database     include "connection.php";     //if user posts called login     if(isset($_post['login'])){         //declaring variables user input , using escape string protect php scripts         $user = mysqli_real_escape_string($dbconn,$_post['user']);         $pass = mysqli_real_escape_string($dbconn,$_post['pass']);         //select users table user input matches un , pw         $sel_user = "select * users un='$user' , pw='$pass'";         //put content held in sel_user variable run_user         $run_user = mysqli_query($dbconn, $sel_user);         //use run_user counting rows , save in check_user         $check_user = mysqli_num_rows($run_user);         //if content row numbers greater 0          if($check_user>0){             //session un equal user input stored in $user             $_session['username']=$user;             //display admin main page             header('location: ../adminmain.php');         }         else {             //display log in error page             header('location: ../loginerror.php');         }     }     //close database connection     mysqli_close($dbconn); ?> 

start session code says undefined variables:

<?php     include"includes/loginrequiredb.php";     if($_session['username'] !=$user){         session_destroy();         header("location: view.php");         die();     }else      {         echo "welcome site have logged in" . $_session['username'];     } ?> 

without starting session can not values $_session.

you need start session in both files as:

session_start(); 

note need start_session() in both files in welcome file.

side note:

i suggest use isset() checking either value set or not.