不在PHP中显示错误消息

如果在注册或登录sit时出现任何错误，则仅打印变量时不会显示错误消息。以下是代码。也尝试执行echo，但也不起作用。尝试将结果存储在变量中并显示消息，但仍不起作用

    <?php 
include 'includes/db.php';
$match = '';
if(isset($_POST['submit_user']))
{
     $email = $_POST['email'];
      $check=mysqli_query($conn,"select * from users where user_email='$email'");
      $checkrows=mysqli_num_rows($check);
      if($checkrows>0) 
      {           
          $msg = "Email Already Exists";
          header('Location:registration.php?msg=$msg');
      } 
      else 
      {
        if($_POST['password'] == $_POST['con_password'])
        {
            $ins_sql = "INSERT INTO users (first_name, last_name, user_email, user_password, username) VALUES ('$_POST[first_name]', '$_POST[last_name]', '$_POST[email]', '$_POST[password]', '$_POST[username]')";
            $run_sql = mysqli_query($conn,$ins_sql);
        }
        else 
        {
            $match = '<div class="alert alert-danger">Password doesn&apos;t match!</div>';
        }
      }

} 

        ?>
                <form class="form-horizontal" action="registration.php" method="post" role="form">
                <div class="body">
                    <div class="row clearfix">
                        <div class="col-sm-6 col-xs-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="text" class="form-control" placeholder="First Name" name="first_name" id="first_name" onkeypress="return checkSpcialChar(event)" required>
                                </div>
                            </div>
                        </div>
                        <div class="col-sm-6 col-xs-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="text" class="form-control" placeholder="Last Name" name="last_name" id="last_name" required>
                                </div>
                            </div>
                        </div>
                    </div>                                                   
                    <div class="row clearfix">
                        <div class="col-sm-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="text" class="form-control" placeholder="Enter Your Email" name="email"  id="email" required>
                                </div>
                            </div>
                        </div>
                         <div class="col-sm-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="password" class="form-control" placeholder="Password" name="password"  id="password" required>
                                </div>
                            </div>
                        </div>
                         <div class="col-sm-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="type" class="form-control" placeholder="Confirm Password" name="con_password"  id="con_password" required>
                                </div>
                            </div>
                        </div>                                            
                        <div class="col-sm-12">
                            <div class="form-group">
                                <div class="form-line">
                                    <input type="text" class="form-control" placeholder="Enter Username" name="username"  id="username">
                                </div>
                            </div>
                        </div>
                        <div class="col-sm-12">
                            <div class="form-group drop-custum">
                                <select class="form-control show-tick" name="user_role" id="user_role" required>
                                    <option value="">-- Select User role --</option>
                                    <option value="admin">Admin</option>
                                    <option value="employee">Employee</option>
                                </select>
                            </div>
                        </div>
                        <div class="col-sm-12">
                            <button type="submit" class="btn btn-raised g-bg-cyan" name="submit_user" id="subject">Submit</button>
                            <button type="submit" class="btn btn-raised">Cancel</button>
                        </div>
                        <span>
                         <?php if(isset($_GET['msg']))
                          echo $_GET['msg'];
                          ?>
                        </span>   
                    </div>
                </div>
                </form>

---

（1）您的代码对SQL注入非常开放，这不仅是一个巨大的安全缺陷，也是常见的bug和问题源。（2） 你将用户密码存储为纯文本，这对你的用户来说是极不负责任的。（3） 你面临的具体问题是什么？描述不清楚。是否有任何错误？此脚本名为
registration.php
$variables
未展开为单个带引号的字符串literal
标题（'Location:registration.php？msg=$msg'）头（“Location:registration.php？msg=$msg”）
退出
也是一个好主意在
标题（“位置：…）
之后，因为
标题（）
函数不会像您所想的那样停止执行