PHP関数を使用して登録フォームの入力を適切に検証するにはどうすればよいですか？

私はPHPの初心者で、登録フォームを検証しようとしていますが、preg_match関数と電子メールフィルター関数が入力を検証していません。フォームが送信され、値がデータベースに挿入され、入力が検証されずに機能します。これが私のPHPです：

<?php

// define errors
$firstnameErr = $lastnameErr = $usernameErr = $passwordErr = $emailaddressErr = "";




if ($_SERVER["REQUEST_METHOD"] == "POST") {


  if (empty($_POST["firstname"])) {
    $firstnameErr = "First Name is required";
  } else {
    $firstname = ($_POST["firstname"]);
    // name only contains letters and space
    if (!preg_match("/^[a-zA-Z ]*$/",$firstname)) {
      $firstnameErr = "Only letters and white space allowed";
    }
  }
    if (empty($_POST["lastname"])) {
    $lastnameErr = "Last Name is required";
  } else {
    $lastname= ($_POST["lastname"]);
    // name only contains letters and space
    if (!preg_match("/^[a-zA-Z ]*$/",$lastname)) {
      $lastnameErr = "Only letters and white space allowed";
    }
  }


 if (empty($_POST["emailaddress"])) {
    $emailaddressErr = "Email address is required";
  } else {
    $emailaddress = ($_POST["emailaddress"]);

    if (!filter_var($emailaddress, FILTER_VALIDATE_EMAIL)) {
      $emailaddressErr = "Invalid email format";
    }
  }


 if (empty($_POST["username"])) {
    $usernameErr = "A username is required";
  } else {
    $username = ($_POST["username"]);
  }



            if (empty($_POST["password"])) {
    $passwordErr = "A password is required";
  } else {

    $password = PASSWORD_HASH($_POST['password'], PASSWORD_DEFAULT);
        }

私のHTMLは以下の通りです。正規表現のために名はクライアント側で検証されると思いますが、preg_matchを使用してサーバー側でも検証する必要があります。私はこれを正しく行っているかどうかわかりません。

<form style="display:flex"; name="signupform" action="registration.php" method="post">


              <div class="container">
                <div class ="row justify-content-center">
                    <div class ="col-md-6">
                        <h1>Registration</h1>
                        <hr class="mb-3">

                        <label for="firstname"><b>First Name</b></label>
                        <input class= "form-control" type="text" placeholder="Enter your First Name" name="firstname" required>
                        <span class="error">* <?php echo $firstnameErr;?></span>

                        <label for="lastname"><b>Last Name</b></label>
                        <input class= "form-control" type="text" placeholder="Enter your Last Name" name="lastname" required>
                        <span class="error">* <?php echo $lastnameErr;?></span>

                        <label for="emailaddress"><b>Email Address</b></label>
                        <input class= "form-control" type="text" placeholder="Enter your Email Address" name="emailaddress" required>
                        <span class="error">* <?php echo $emailaddressErr;?></span>


                        <label for="username"><b>Username</b></label>
                        <input class= "form-control" type="text" placeholder="Enter your desired username" name="username" required>
                        <span class="error">* <?php echo $usernameErr;?></span>

                        <label for="password"><b>Password</b></label>
                        <input class= "form-control" type="password" placeholder="Enter a password" name="password" pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,}" title="Must contain at least one number and one uppercase and lowercase letter, and at least 8 or more characters" required>
                        <span class="error">* <?php echo $passwordErr;?></span>
                        <em> Password must contain at least one number and one uppercase and lowercase letter, and at least 8 or more characters</em><br>


                        <hr class ="mb-3">
                        <input class="btn btn-block btn-primary" type="submit" name="create" value="Sign Up">
                        <hr class = "mb-3">


                    </div>

                </div>
            </div>  
    </form>