php 验证特殊字符

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

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

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

    $stmt->execute();

    $result = $stmt->fetchAll();
    foreach ($result as $row) {
        echo $row['username'] . "<br>";
    }
} catch(PDOException $e) {
    echo "Error: " . $e->getMessage();
}
$conn = null;
?>