重定向问题而不是正确的细节

<?php

session_start();

// Including header/ conifg file.
include("include/header.php");

if (isset($_POST['login']))
 {      // Check login is submit or not.

 $email = $_POST['user_name'];

$password = $_POST['password'];

    $query = "SELECT `id`,`email_id`, `password`, `first_name`,`roles` FROM tbl_registration WHERE email_id='" . $email . "' AND password='" . $password . "'";

    $result = mysqli_query($conn, $query);

if ($num > 0)
 {

$row = mysql_fetch_assoc($result);

if ($row['role'] == 1) {

$_SESSION["user_id"] = $row["id"];

$_SESSION["first_name"] = $row["first_name"];

 $_SESSION["email_id"] = $row["email_id"];

header("Location: admin/index.php");

} 
else {

 $_SESSION["user_id"] = $row["id"];

 $_SESSION["first_name"] = $row["first_name"];

 $_SESSION["email_id"] = $row["email_id"];

 header("Location: index.php");

  }

 }

}

?>

1. It seems like your if-statement isn't running because the variable $num isn't set. You need to add:
   $num = mysqli_num_rows($result);

2. You have misspelled role and roles in if ($row['role'] == 1) and column name `roles`

3. Substitute mysql with mysqli when fetching result, writing instead:
   $row = mysqli_fetch_assoc($result);

4. Make sure to provide information for $conn:
   $conn = mysqli_connect("host", "user", "pass", "db");
   
   

   <?php
   
   session_start();
   
   // Including header/ conifg file.
   include("include/header.php");
   
   // Check login is submit or not.
   if (isset($_POST['login']))
    {  
       $conn = mysqli_connect("host", "user", "pass", "db");
   
       if (mysqli_connect_errno()) {
       printf('Connect failed: %s
   ', mysqli_connect_error());
       exit();
       }    
   
       $email = $_POST['user_name'];
       $password = $_POST['password'];
       $query = "SELECT `id`,`email_id`, `password`, `first_name`,`roles` FROM tbl_registration WHERE email_id='" . $email . "' AND password='" . $password . "'";
       $result = mysqli_query($conn, $query);
       $num = mysqli_num_rows($result);
   
   if ($num > 0)
       {
   
       $row = mysqli_fetch_assoc($result);
   
       if ($row['roles'] == 1) {
   
          $_SESSION["user_id"] = $row["id"];
   
          $_SESSION["first_name"] = $row["first_name"];
   
          $_SESSION["email_id"] = $row["email_id"];
   
          header("Location: admin/index.php");
   
               } 
       else {
   
       $_SESSION["user_id"] = $row["id"];
   
       $_SESSION["first_name"] = $row["first_name"];
   
       $_SESSION["email_id"] = $row["email_id"];
   
       header("Location: index.php");
   
          }
   
       }
   
   }
   
   ?>