給你一個極度簡易的示意範例

front.php

<form action = "./login.php" method = "post">
  <label for = "account">Account</label>
  <input type = "text" id = "account" name = "maccount"><br>
  <label for = "password">Password</label>
  <input type = "password" id = "password" name = "mpassword"><br>
  <button type = "submit">Login</button>
</form>

<style>
  * {
    margin: 5px;
  }
  body {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
  }
</style>

login.php

<?php
  $users = array(
    array("account" => "user1", "password" => "123", "mission" => "1"),
    array("account" => "user2", "password" => "123", "mission" => "2"),
    array("account" => "user3", "password" => "123", "mission" => "3"),
  );
  if (isset($_POST["maccount"]) && isset($_POST["mpassword"]) ) {
    $account = $_POST["maccount"];
    $password = $_POST["mpassword"];
    $userData = null;
    foreach($users as $key => $val) {
      if ($val['account'] == $account) {
        $userData = $users[$key];
      }
    }
    if (!$userData) {
      echo "login fail";
      return;
    }
    if ($password == $userData['password']) {
      switch ($userData['mission']) {
        case '1': {
          header( 'Location: 1.php' );
          break;
        }
        case '2': {
          header( 'Location: 2.php' );
          break;
        }
        case '3': {
          header( 'Location: 3.php' );
          break;
        }
        default: {
          break;
        }
      }
    } else {
      echo "login fail";
    }
  }
?>

共有三個 user

分別有不同權限

登入後會導向各身分的頁面

這只是示意

不同身分來導向不同頁面而已

至於那些

連資料庫

密碼不該以明文儲存

session 之類的

這裡不考慮