对于一个相对简单的过程，你有很多行代码。就个人而言，可以通过添加 required 在每个html表单输入元素的末尾（这就是我要做的）
其次，md5对于散列密码是不安全的（你是散列密码而不是加密密码）
第三，这里有一种使用bcrypt对表单中的密码进行散列的方法，它比使用md5散列要好得多。所以在检查错误之前要做什么 like counting the usernames and if row > 0 die('username exists) 使用pdo的基本完整代码示例
当检查用户登录时，只需使用 password_verify() 这样做的功能
整洁的代码可以帮助人们理解你的问题所在，并且通常更易于阅读。我知道你可能只是在寻找一些'做的工作'，但它可以帮助你调试时，我们当你要求帮助。
我会给你一个比你的方法更安全的方法。
索引.php

<form method="post" id="regform" action="register.php">
    <input type="text" name="username" placeholder="Enter your email    Address"required/>
    <input type="password" name="password" placeholder="Enter your password" required/>
    <input type="submit" class="indexbttn" id="indexbttn" name="enter"value="enter"/>
</form>

连接.php

<?php
$servername = "localhost";
$dbusername = "root";
$dbpassword = "root";
$dbname = "fyp";
try{
$pdo = new PDO("mysql:host=$servername;dbname=$dbname",$dbusername,   $dbpassword);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e)
{
print "Error! Unable to connect: " . $e->getMessage() . "<br/>";
die();
}
?>

寄存器.php

<?php
session_start();
require_once ('connect.php');
error_reporting(E_ALL);
ini_set('display_errors', 1);

 if(isset($_POST['enter'])){
  $username = !empty($_POST['username']) ? trim($_POST['username']) : null;
  $pass = !empty($_POST['password']) ? trim($_POST['password']) : null;
  $check (!filter_var($_POST['username'], FILTER_VALIDATE_EMAIL));

  $cnt = "SELECT COUNT(username) AS num FROM users WHERE username = :username";
  $stmt = $pdo->prepare($cnt);
  $stmt->bindValue(':username', $username);
  $stmt->execute();
  $row = $stmt->fetch(PDO::FETCH_ASSOC);
  if($row['num'] > 0){
      die('That username already exists!');
  }
  $passHash = password_hash($pass, PASSWORD_BCRYPT, array("cost" => 12));
  $insrt = "INSERT INTO users (username, password) VALUES (:username, :password)";
  $stmt = $pdo->prepare($insrt);
  $stmt->bindValue(':username', $username);
  $stmt->bindValue(':password', $passHash);
  $result = $stmt->execute();
  if($result){
    header( "refresh:5;url=index.php" );
echo 'You will be redirected in  5 seconds. If not, click <a       href="index.php">here</a>.';
  }
}
?>

登录.php

<?php
 session_start();
require("connect.php");
  if(isset($_POST['enter'])){
$username = !empty($_POST['username']) ? trim($_POST['username']) :   null;
 $pass = !empty($_POST['password']) ? trim($_POST['password']) : null;
$rtrv = "SELECT username, password, userid FROM users WHERE username =       :username";
  $stmt = $pdo->prepare($rtrv);
  //Bind value.
  $stmt->bindValue(':username', $username);
  //Execute.
  $stmt->execute();
  //Fetch row.
   $user = $stmt->fetch(PDO::FETCH_ASSOC);
  //If $row is FALSE.
  if($user === false){
  //Could not find a user with that username!
  die('Incorrect username');
  }
  else{
      $validPassword = password_verify($pass, $user['password']);
    if($validPassword){
        $_SESSION['user_id'] = $user['username'];
        $_SESSION['logged_in'] = time();
        header( "Location: /protected.php" );
        die();
      } else{
          die('Wrong password!');
    }
   }
 }
 ?>