PHP唯一文本字段？

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(255) UNIQUE NOT NULL,
    email VARCHAR(255) UNIQUE NOT NULL,
    password VARCHAR(255) NOT NULL
);

$conn = new mysqli($servername, $username, $password, $dbname);

$sql = "INSERT INTO users (username, email, password) VALUES ('$username', '$email', '$password')";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();