Php 返回的记录数。在这种情况下，OP绝对不需要它。@Dharman好的，谢谢。我给它添加了另一个选项。我同意有不止一种方法可以用来检查PDO中是否存在记录。我不明白为什么不调用它。最后两条$msg语句在我用代码创建新帐户时出现$msg在用echo语句重新加载时

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返回的记录数。在这种情况下，OP绝对不需要它。@Dharman好的，谢谢。我给它添加了另一个选项。我同意有不止一种方法可以用来检查PDO中是否存在记录。我不明白为什么不调用它。最后两条

$msg

语句在我用代码创建新帐户时出现

$msg

在用

echo

语句重新加载时，在HTML表单中被调用。@JeremyMyrtle自从我最初发表文章以来，我做了很多编辑。请重新加载它。PHP代码中只有一个标题，我已经在其中添加了

exit（）

。我还是不懂

$msg

。它在PHP中作为一个空白变量启动。然后，如果发生错误，将重写

$msg

以显示消息。之后，需要刷新页面并显示错误消息。这发生在PHP代码中的最后两个

$msg

上。我必须指出，

rowCount

很少是一个好的选择。您通常不需要知道返回记录的数量。在这种情况下，OP绝对不需要它。@Dharman好的，谢谢。我给它添加了另一个选项。我同意有不止一种方法可以用来检查PDO中是否存在记录。我不明白为什么不调用它。最后两条

$msg

语句在我用代码创建新帐户时出现

$msg

在用

echo

语句重新加载时，在HTML表单中被调用。@JeremyMyrtle自从我最初发表文章以来，我做了很多编辑。请重新加载它。PHP代码中只有一个标题，我已经在其中添加了

exit（）

。我还是不懂

$msg

。它在PHP中作为一个空白变量启动。然后，如果发生错误，将重写

$msg

以显示消息。之后，需要刷新页面并显示错误消息。这发生在PHP代码中的最后两个

$msg

上。

<?php

session_start();

include('api/dbconnect.php');

$msg = "";

if(isset($_POST['register-submit'])) {
// Now we check if the data was submitted, isset() function will check if the data exists.
    if (!isset($_POST['first_name'], $_POST['last_name'], $_POST['email'], $_POST['username'], $_POST['password'])) {
        // Could not get the data that should have been sent.
        $msg = 'Please complete the registration form';
    }
// Make sure the submitted registration values are not empty.
    if (empty($_POST['first_name']) || 
        empty($_POST['last_name'])  || 
            /* empty($_POST['email']) || */ 
        empty($_POST['username']) || 
        empty($_POST['password'])) {
        
        // One or more values are empty.
        $msg = 'All fields are required for the form to be submitted';
    }

    /*if (!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
        die ('The email address provided is invalid');
    }*/

    if (preg_match('/[A-Za-z0-9]+/', $_POST['username']) == 0) {
        $msg = 'The username provided is invalid';
    }

    if (strlen($_POST['password']) > 20 || strlen($_POST['password']) < 5) {
        $msg = 'Password must be between 5 and 20 characters';
    }

    if ($_POST['password'] != $_POST['confirm_pwd']) {
        $msg = 'The two password provided do not match.';
    }

    // We need to check if the account with that username exists.
    if ($stmt = $pdo->prepare('SELECT id, password FROM users WHERE username = ?')) {
        // Bind parameters (s = string, i = int, b = blob, etc), hash the password using the PHP password_hash function.
        $stmt->bindParam(1, $_POST['username']);
        $stmt->execute();
        // Store the result so we can check if the account exists in the database.
        if ($stmt->num_rows > 0) {
            // Username already exists
            $msg = 'Another account with this username already exists';
        } else {
            // Username doesn’t exist, insert new account
            if ($stmt = $pdo->prepare('INSERT INTO users (first_name, last_name, email, username, password) VALUES (?, ?, ?, ?, ?)')) {
                // We do not want to expose passwords in our database, so hash the password and use password_verify when a user logs in.
                $password = password_hash($_POST['password'], PASSWORD_DEFAULT);
                $stmt->bindParam(1, $_POST['first_name']);
                $stmt->bindParam(2, $_POST['last_name']);
                $stmt->bindParam(3, $_POST['email']);
                $stmt->bindParam(4, $_POST['username']);
                $stmt->bindParam(5, $password);
                $stmt->execute();
                header('Location: users.php');
            } else {
                // Something is wrong with the SQL statement. Check to make sure the users table exists with all three fields.
                $msg = 'There was a problem creating this account. Contact your Network Administrator.';
            }
        }
        $stmt->close();
    }
    else {
        // Something is wrong with the SQL statement. Check to make sure the users table exists with all 3 fields.
        $msg = 'There was a problem creating this account. Contact your network administrator.';
    }
    //$con->close();
}

?>

<form class="user" method="post" action="user_new.php">
  <div class="custom-control small">
    <strong class="text-danger"><?php echo $msg; ?></strong>
  </div>
  <div class="form-group row">
    <div class="col-sm-6 mb-3 mb-sm-0">
      <input type="text" class="form-control form-control-user" id="first_name" name="first_name" placeholder="First Name" autocomplete="off" required>
    </div>
    <div class="col-sm-6">
      <input type="text" class="form-control form-control-user" id="last_name" name="last_name" placeholder="Last Name" autocomplete="off" required>
    </div>
  </div>
  <div class="form-group row">
    <div class="col-sm-6 mb-3 mb-sm-0">
      <input type="email" class="form-control form-control-user" id="email" name="email" placeholder="Email Address (optional)" autocomplete="off">
    </div>
    <div class="col-sm-6">
      <input type="text" class="form-control form-control-user" id="username" name="username" placeholder="Username" autocomplete="off" required>
    </div>
  </div>
  <div class="form-group row">
    <div class="col-sm-6 mb-3 mb-sm-0">
      <input type="password" class="form-control form-control-user" id="password" name="password" placeholder="Password" autocomplete="off" required>
    </div>
    <div class="col-sm-6">
      <input type="password" class="form-control form-control-user" id="confirm_pwd" name="confirm_pwd" placeholder="Confirm Password" autocomplete="off" required>
    </div>
  </div>
  <button type="submit" name="register-submit" class="btn btn-primary btn-user btn-block" style="background-color: #a40000; border-color: #a40000;">Create User Account</button>
  <hr>
  <a href="users.php" class="btn btn-warning btn-user btn-block" style="background-color: #2658a8; border-color: #2658a8;">Return to Users</a>
</form>

 $if_row = $stmt->fetchColumn();

 if ($if_row > 0) {
     $msg = "A record exists.";
 }

<?php if(!empty($msg)) { echo $msg; } ?>