It should be if($logedin_admin=='1'){
You are doing an assignment operation instead of a comparison..
Also, make sure if the session was started...
The code..
<?php
session_start(); //<=--------- Add this
if($logedin_admin=='1'){
$logedin_admin=0;
$_SESSION['is_admin']=1;
header('Location: admin/index.php');
exit; //<=----------- Add an exit too.
}