此电子邮件表单验证 php 代码是否有效？

Question

我有一个包含 html 电子邮件表单的 php 文件，并且我有位于 html 代码之上的服务器端表单验证。我只有基础知识，我想知道是否有人能够判断下面的验证代码是否正确（我还在代码中插入了一些问题）。谢谢！

<?php
// define variables and set to empty values
$nameErr = $emailErr = $messageErr = $keywordErr = "";
$name = $email = $message = $keyword = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    if (empty($_POST["name"])) {
        $nameErr = "Name is required";
    } else {
        $name = test_input($_POST["name"]);
        // check if name only contains specific names - is this ok?
        if (!preg_match("/^[Apollo, Mars, Venus, Sofia]*$/", $name)) {
            $nameErr = "Only preset names allowed";
        }
    }
    if (empty($_POST["email"])) {
        $emailErr = "Email is required";
    } else {
        $email = test_input($_POST["email"]);
        // check if e-mail address is well-formed - is this ok and where does the "FILTER_VALIDATE_EMAIL" reside?
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $emailErr = "Invalid email format";
        }
    }
    if (empty($_POST["message"])) {
        $messageErr = "Message is blank";
    } else {
        $message = test_input($_POST["message"]);
    }
    if (empty($_POST["keyword"])) {
        $nameErr = "Answer is required";
    } else {
        $name = test_input($_POST["keyword"]);
        // check if answer matching - is expression correct?
        if (!preg_match("/^[Actual answer words]*$/", $keyword)) {
            $keywordErr = "Incorrect answer. Please try again.";
        }
    }
}
function test_input($data)
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);

    return $data;
}

我根据“W3学校”的php代码教程检查了代码，但我不清楚它是否足够安全，以及我尝试自定义的表达式以及我有问号的地方。我还计划在文件中进行 html 客户端验证，但这是另一个问题。