HTTP 501 Not Implemented

What is HTTP 501 Not Implemented?

The 501 (Not Implemented) status code indicates that the server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.

Common Use Cases

• Unsupported HTTP methods
• Server missing required functionality
• Legacy servers receiving modern request features

// PHP - returning 501 for unsupported methods
$method = $_SERVER['REQUEST_METHOD'];
if (!in_array($method, ['GET', 'POST'])) {
    http_response_code(501);
    echo json_encode(['error' => "Method $method not implemented"]);
    exit;
}