PHP 양식 처리의 보안 취약점을 방지하는 방법

<?php
$name = htmlspecialchars($_POST['name']);
echo "<p>欢迎，" . $name . "！</p>";
?>

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$stmt = $conn->prepare("SELECT * FROM users WHERE username = :username");
$stmt->bindParam(':username', $_POST['username']);
$stmt->execute();

$result = $stmt->fetch(PDO::FETCH_ASSOC);
if($result){
    echo "欢迎，" . $result['username'] . "！";
} else {
    echo "用户不存在！";
}
?>

<?php
$targetDir = "uploads/";
$targetFile = $targetDir . uniqid() . '_' . basename($_FILES['file']['name']);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($targetFile,PATHINFO_EXTENSION));

// 验证文件类型
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
    && $imageFileType != "gif" ) {
    echo "只允许上传 JPG, JPEG, PNG 和 GIF 格式的文件!";
    $uploadOk = 0;
}

// 验证文件大小
if ($_FILES["file"]["size"] > 500000) {
    echo "文件大小不能超过 500KB!";
    $uploadOk = 0;
}

// 上传文件
if ($uploadOk == 0) {
    echo "文件上传失败.";
} else {
    if (move_uploaded_file($_FILES["file"]["tmp_name"], $targetFile)) {
        echo "文件上传成功：". $targetFile;
    } else {
        echo "文件上传失败.";
    }
}
?>