PHP 表单 - 相同的网页验证，加上插入数据库

我正在设计一个带有PHP PDO的注册表单，供用户插入其中的信息，然后通过PHPMyAdmin上传到数据库，所有这些都在同一个PHP网页中。

有了这个，它就可以工作，验证也可以工作。 但我遇到的问题是，即使存在验证错误，例如，在用户点击"提交"后，所有字段都是空白的。表单仍然通过，然后在数据库中插入一个空白行。

我不明白为什么表单将信息插入数据库，即使有错误。

<?php
    ini_set("display_errors", 1);
    error_reporting(E_ALL);
        // First we execute our common code to connection to the database and start the session 
        require("common.php");
        $usernameErr = $emailErr = $passwordErr = $password1Err = "";
        $username = $email = $password = "";
        ///////////////////////////////////////////////////////////////////
        if(!empty($_POST)) 
        { 
        $usernamePOST = $_POST['username'];
        $emailPOST = $_POST['email'];
        $passwordPOST = $_POST['password'];
        $password1POST = $_POST['password1'];
            // Email validation
            if (empty($_POST["email"])) {
             $emailErr = "<p class='errorm'>Email is required</p>";
           } else {
             $email = test_input($_POST["email"]);
             // check if e-mail address is well-formed
             if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
               $emailErr = "<p class='errorm'>Invalid email format</p>";
             }
           }
           //if (!filter_var($email, FILTER_VALIDATE_EMAIL)){
                //echo "<p class='errorm'>Please enter a valid email address</p>";
            //}
            ///////////////////////////////////////////////////////////////////
            // Username validation
            // Make sure the user entered a username 
            if (strlen($username) <= 6){
                $usernameErr = "<p class='errorm'>Choose a Username longer then 7 characters</p>";
            }
            if (empty($_POST["username"])) {
             $usernameErr = "<p class='errorm'>Username is required</p>";
           } else {
             $username = test_input($_POST["username"]);
             // check if name only contains letters and whitespace
             if (!preg_match("/^[a-zA-Z ]*$/",$username)) {
               $usernameErr = "<p class='errorm'>Only letters allowed</p>";
             }
           }
            ///////////////////////////////////////////////////////////////////
            // Password validation
           if (empty($_POST["password"])) {
             $passwordErr = "<p class='errorm'>Password is required</p>";
           } else {
             $password = test_input($_POST["password"]);
           }
           // Password match
            if ($_POST["password"] != $_POST["password1"]){
                $password1Err = "<p class='errorm'>Passwords in both fields, don't match</p>";
            }
            // Password length
            if (strlen($password) <= 5){
                $passwordErr = "<p class='errorm'>Choose a Password longer then 6 characters</p>";
            }
            ///////////////////////////////////////////////////////////////////
            function test_input($data) 
            {
               $data = trim($data);
               $data = stripslashes($data);
               $data = htmlspecialchars($data);
               return $data;
            }
            ///////////////////////////////////////////////////////////////////
        if(!isset($error)){
        //no error
        $sthandler = $db->prepare("SELECT username FROM users WHERE username = :username");
        $sthandler->bindParam(':username', $username);
        $sthandler->execute();
        if($sthandler->rowCount() > 0){
            header("refresh:10;url=index.php" );
            echo "<p>Sorry, this Username already exists<p>";
            echo '<p>You''ll be redirected back to the Register page in about 10 secs. If this does not happen, please click <a href="index.php">here</a></p>';
            //$errmsg_arr[] = "Username Already Exists";
            //$errflag = true;
        } else {
            //Securly insert into database
            $sql = 'INSERT INTO users (username, email, password) VALUES (:username,:email,:password)';
            $query = $db->prepare($sql);
            $query->execute(array(
            ':username' => $_POST['username'], ':email' => $_POST['email'], ':password' => $_POST['password']));
            }
        }
    }
?>