Build a REST API from Scratch: An Introduction

The current internet ecosystem has literally been invaded by APIs, and for good reasons. By using third party APIs in your products or services, you have access to a ton of useful features — such as authentication or storage services — that can benefit both you and your users. By exposing your own API, your application becomes “part of the mix” and will be used in ways you’ve never thought before… if you do it the right way, obviously. In this two part series I’ll show you how to create a RESTful API layer for your PHP applications, using a collection of real world best practices. The full source code of this project will be available at the end of part 2.

REST: A pleasant UI for developers

First of all, an API is a user interface for developers, so it must be friendly, simple, easy to use and of course pleasant; or else it will end up being another piece of digital junk out there. Documentation, even in the form of a simple but well written README file, is a good place to start. The minimal information we need is a summary of the service’s scope and the list of methods and access points. A good summary can be:

Our application is a simple contact list service that manages contacts with linked notes. It has two object types, contacts and notes. Each contact has basic attributes such as first name, last name, and email address. Also, each contact can have a number of markdown-formatted notes linked to it.

Then, it’s a good idea to make a list of all the resources and actions that we are going to implement. This can be seen as the equivalent of wireframing for visual applications. Following the key principles of REST, each resource is represented by a URL, where the action is the HTTP method used to access it. For example GET /api/contacts/12 retrieves the contact with id of 12, while PUT /api/contacts/12 will update that same contact. The full list of methods is displayed below:

URL HTTP Method Operation
/api/contacts GET Returns an array of contacts
/api/contacts/:id GET Returns the contact with id of :id
/api/contacts POST Adds a new contact and return it with an id attribute added
/api/contacts/:id PUT Updates the contact with id of :id
/api/contacts/:id PATCH Partially updates the contact with id of :id
/api/contacts/:id DELETE Deletes the contact with id of :id

/api/contacts/:id/star PUT Adds to favorites the contact with id of :id
/api/contacts/:id/star DELETE Removes from favorites the contact with id of :id

/api/contacts/:id/notes GET Returns the notes for the contact with id of :id
/api/contacts/:id/notes/:nid GET Returns the note with id of :nid for the contact with id of :id
/api/contacts/:id/notes POST Adds a new note for the contact with id of :id
/api/contacts/:id/notes/:nid PUT Updates the note with id if :nid for the contact with id of :id
/api/contacts/:id/notes/:nid PATCH Partially updates the note with id of :nid for the contact with id of :id
/api/contacts/:id/notes/:nid DELETE Deletes the note with id of :nid for the contact with id of :id

For a more complete and professional documentation, you may consider some tools like Swagger, apiDoc or Google APIs Discovery Service: your users will love you!

Tools and setup

The main tool I’m going to use to build our API is Slim Framework. Why?

[It] helps you quickly write simple yet powerful web applications and APIs.

And that’s true. Its powerful routing feature makes it easy to use methods other than the standard GET and POST, it provides built-in support for HTTP method overridess (via both HTTP header and hidden POST fields) and it can be hooked with middleware and extra features to make apps and API development really easy. Along with Slim I’m using Idiorm to access the database layer and Monolog for logging. So our composer.json file will look like this:

{
"name": "yourname/my-contacts",
"description": "Simple RESTful API for contacts management",
"license": "MIT",
"authors": [
{
"name": "Your Name",
"email": "you@yourdomain.com"
}
],
"require": {
"slim/slim": "*",
"slim/extras": "*",
"slim/middleware": "*",
"monolog/monolog": "*",
"j4mie/paris": "*",
"flynsarmy/slim-monolog": "*"
},
"archive": {
"exclude": ["vendor", ".DS_Store", "*.log"]
},
"autoload": {
"psr-0": {
"API": "lib/"
}
}
}

The slim/extras and slim/middleware packages provide useful features such as content type parsing and basic authentication. Our custom classes are under the namespace of API and sit inside the lib directory. At this point our working directory structure would look like this:

bootstrap.php
composer.json
README.md
bin/
    import
    install
lib/
    API/
public/
    .htaccess
    index.php
share/
    config/
        default.php
    db/
    logs/
    sql/
        data/
            contacts.sql
            users.sql
        tables/
            contacts.sql
            notes.sql
            users.sql
        ssl/
            mysitename.crt
            mysitename.key

The front controller of our application is public/index.php, where all the non-file-or-directory traffic is redirected via standard URL rewrite rules. I’ve then placed all the initialization code in bootstrap.php which we’ll see later. The share directory contains data such as logs, configuration files, the SQLite databases and dump files, and the SSL certificates. The bin directory contains utility scripts that create the database and import some data using the provided .sql files.

SSL Everywhere

Our API will be accessible only in HTTPS mode, with no redirection. This simplifies the authentication logic and prevents poorly configured clients to access non encrypted endpoints. The easiest and more logical way to set this up is acting directly on the web server or through a proxy server. I’m using old trusted Apache to do this, and my the virtual host file looks like this:

<Directory "/path/to/MyApp/public">

# Required for mod_rewrite in .htaccess
AllowOverride FileInfo

Options All -Indexes

DirectoryIndex index.php index.shtml index.html

<IfModule php5_module>
# For Development only!
php_flag display_errors On
</IfModule>

# Enable gzip compression
<ifModule filter_module>
AddOutputFilterByType DEFLATE application/json
</ifModule>

Order deny,allow
Deny from all
Allow from 127.0.0.1
</Directory>

<VirtualHost *:80>
ServerAdmin you@yourdomain.com
DocumentRoot "/path/to/MyApp/public"
ServerName myapp.dev

<IfModule rewrite_module>
RewriteEngine on

## Throw a 403 (forbidden) status for non secure requests
RewriteCond %{HTTPS} off
RewriteRule ^.*$ - [L,R=403]
</IfModule>

</VirtualHost>

<IfModule ssl_module>

NameVirtualHost *:443

Listen 443
SSLRandomSeed startup builtin
SSLRandomSeed connect builtin

<VirtualHost *:443>
ServerAdmin you@yourdomain.com
DocumentRoot "/path/to/MyApp/public"
ServerName myapp.dev

SSLEngine on
SSLCertificateFile /path/to/MyApp/share/ssl/mysitename.crt
SSLCertificateKeyFile /path/to/MyApp/share/ssl/mysitename.key

SetEnv SLIM_MODE development

</VirtualHost>
</IfModule>

Directory settings are defined first, so that they are in common with both the HTTP and HTTPS version of our site. In the non-secure host configuration I’m using mod_rewrite to issue a 403 Forbidden error for any non secure connection, then in the secure section I’m setting up SSL using my self-signed certificates, along with the SLIM_ENV variable that tells Slim the current application mode. For more information on how to create a self signed certificate an install it on your Apache see this article on SSLShopper. Now that we have clear objectives, a basic directory structure, and a server setup, let’s run a composer.phar install and start writing some code.

Bootstrap and Front Controller

As said before, the bootstrap.php file is responsible for loading our application settings and autoloader setup.

// Init application mode
if (empty($_ENV['SLIM_MODE'])) {
$_ENV['SLIM_MODE'] = (getenv('SLIM_MODE'))
? getenv('SLIM_MODE') : 'development';
}

// Init and load configuration
$config = array();

$configFile = dirname(__FILE__) . '/share/config/'
. $_ENV['SLIM_MODE'] . '.php';

if (is_readable($configFile)) {
require_once $configFile;
} else {
require_once dirname(__FILE__) . '/share/config/default.php';
}

// Create Application
$app = new API\Application($config['app']);

First I get the current environment. If a file named <EnvName>.php is present it’s loaded, if not the default config file is loaded. Slim specific settings are stored in the $config['app'] array and passed to the constructor of our application that extends the basic Slim object (optional but recommended). For example the statement:

$config['app']['log.writer'] = new \Flynsarmy\SlimMonolog\Log\MonologWriter(array(
'handlers' => array(
new \Monolog\Handler\StreamHandler(
realpath(__DIR__ . '/../logs')
.'/'.$_ENV['SLIM_MODE'] . '_' .date('Y-m-d').'.log'
),
),
));

configures a Monolog logger that writes to a file at app/path/share/logs/EnvName_YYYY-mm-dd.log. Then, after some refinements (you can see them in the source), I get the generated log writer and try to connect to the database:

// Get log writer
$log = $app->getLog();

// Init database
try {

if (!empty($config['db'])) {
\ORM::configure($config['db']['dsn']);
if (!empty($config['db']['username']) && !empty($config['db']['password'])) {
\ORM::configure('username', $config['db']['username']);
\ORM::configure('password', $config['db']['password']);
}
}

} catch (\PDOException $e) {
$log->error($e->getMessage());
}

// Add middleware
// Your middleware here...
$app->add(new Some\Middleware\Class(...));

Lastly I add the needed middleware to my application instance. Slim’s middleware is like the layers of an onion, the first middleware you add will be the innermost layer, so the order of our middleware is important. I’m using the following middleware in our API:

• Cache (the innermost);
• ContentTypes: parses JSON-formatted body from the client;
• RateLimit: manages users API limits;
• JSON: utility middleware for “JSON only responses” and “JSON encoded bodies” best practices;
• Authentication (outermost).

We will write all these except for the pre-existing ContentTypes. At the end of the bootstrap file I’ve defined the two global variables $app (the application instance) and $log (the log writer). The file is loaded by our front controller index.php where the magic happens.

Routing structure

Slim has a nice feature called Route Groups. With this feature we can define our application routes like this:

// API group
$app->group('/api', function () use ($app, $log) {

// Version group
$app->group('/v1', function () use ($app, $log) {

// GET route
$app->get('/contacts/:id', function ($id) use ($app, $log) {

});

// PUT route, for updating
$app->put('/contacts/:id', function ($id) use ($app, $log) {

});

// DELETE route
$app->delete('/contacts/:id', function ($id) use ($app, $log) {

});

});

});

I’ve created two nested groups, /api and /v1, so we can easily adhere to the “versioning in the URL” best practice. I’ve also created some optional routes for /api/, that could contain user-readable content, and a generic root URL (/) URL that in the real world could contain the public user interface for the app.

JSON middleware

My initial approach here was using a route middleware (another type of Slim’s middleware) inside the /v1 group for authentication and JSON requests/responses, but I’ve found it is more practical and clean to use classic middleware. As seen previously, a middleware is an instance of a class that inherits from Slim\Middleware. The call() method of a Slim middleware is where the action takes place, it’s executed automatically when the middleware is linked as global, with the $app->add() method.

$app->add(new API\Middleware\JSON('/api/v1'));

Our JSON middleware achieves two best practices: “JSON only responses” and “JSON encoded bodies”. Here’s how:

// JSON middleware call
public function call()
{
if (preg_match('|^' . $this->root . '.*|', $this->app->request->getResourceUri())) {

// Force response headers to JSON
$this->app->response->headers->set(
'Content-Type',
'application/json'
);

$method = strtolower($this->app->request->getMethod());
$mediaType = $this->app->request->getMediaType();

if (in_array(
$method,
array('post', 'put', 'patch')
)) {

if (empty($mediaType)
|| $mediaType !== 'application/json') {
$this->app->halt(415);
}
}
}
$this->next->call();
}

We can pass a root path to our middleware constructor. In this case I’m passing /api/v1 so that our middleware is applied only on the API part of our site. If the current path matches the response content type header is forced to application/json, then I check the request method. If the request method is one of the write-enabled ones (PUT, POST, PATCH) the request content type header must be application/json , if not the application exits with a 415 Unsupported Media Type HTTP status code. If all is right the statement $this->next->call() runs the next middleware in the chain.

Authentication

Since our application will run on HTTPS by default, I decided to use the token-over-basic-authentication approach: an API key is sent in the username field of the basic HTTP AUTH headers (no password required). In order to do this I wrote a Slim middleware class called TokenOverBasicAuth, by modifying the existing Slim HttpBasicAuth. This middleware is run first in the chain so it is added as last, and it takes an optional root path parameter in the constructor.

// Auth middleware call
public function call()
{
$req = $this->app->request();
$res = $this->app->response();

if (preg_match('|^' . $this->config['root'] . '.*|', $req->getResourceUri())) {

// We just need the user
$authToken = $req->headers('PHP_AUTH_USER');

if (!($authToken && $this->verify($authToken))) {
$res->status(401);
$res->header(
'WWW-Authenticate',
sprintf('Basic realm="%s"', $this->config['realm'])
);
}

}

$this->next->call();
}

The method searches for the auth token within the PHP_AUTH_USER request header, if it does not exist or is invalid a 401 Forbidden status and authentication headers are passed to the client. The verify() method is protected so that it can be overridden by child classes; my version here is simple:

protected function verify($authToken)
{
$user = \ORM::forTable('users')->where('apikey', $authToken)
->findOne();

if (false !== $user) {
$this->app->user = $user->asArray();
return true;
}

return false;
}

Here I’m simply checking for the presence of the API key in the users table, and If I find a valid user it’s added to the application context to be used with the next layer (RateLimit). You can modify or extend this class to inject your own authentication logic or use an OAuth module. For more info on OAuth see Jamie Munro’s article.

Consumable error payloads

Our API should show useful error messages in consumable format, if possible in JSON representation. We need a minimal payload that contains an error code and message. In addition, validation errors require more breakdown. With Slim we can redefine both 404 errors and server errors with the $app->notFound() and $app->error() method respectively.

$app->notFound(function () use ($app) {

$mediaType = $app->request->getMediaType();

$isAPI = (bool) preg_match('|^/api/v.*$|', $app->request->getPath());

if ('application/json' === $mediaType || true === $isAPI) {

$app->response->headers->set(
'Content-Type',
'application/json'
);

echo json_encode(
array(
'code' => 404,
'message' => 'Not found'
)
);

} else {
echo '<html>
<head><title>404 Page Not Found</title></head>
<body><h1>404 Page Not Found</h1><p>The page you are
looking for could not be found.</p></body></html>';
}
});

Not found errors are simpler: first I’m grabbing the media type of the request, then the $isAPI flag tells me if the current URL is under the /api/v* group. If the client requested an API URL or sent a JSON content type header I’m returning a JSON output, else I can render a template or simply print some static HTML, as in this example. Other errors are a little bit tricky, the $app->error() method is triggered when there is an exception and Slim transforms standard PHP errors in ErrorException objects. We need a way to give a useful error to the client without exposing too much of our internal mechanism in order to avoid security flaws. I’ve created two custom exceptions for this application, an API\Exception and an API\Exception\ValidationException that are exposed to the public, all other exception types are logged, and displayed only in development mode.

$app->error(function (\Exception $e) use ($app, $log) {

$mediaType = $app->request->getMediaType();

$isAPI = (bool) preg_match('|^/api/v.*$|', $app->request->getPath());

// Standard exception data
$error = array(
'code' => $e->getCode(),
'message' => $e->getMessage(),
'file' => $e->getFile(),
'line' => $e->getLine(),
);

// Graceful error data for production mode
if (!in_array(
get_class($e),
array('API\\Exception', 'API\\Exception\ValidationException')
)
&& 'production' === $app->config('mode')) {
$error['message'] = 'There was an internal error';
unset($error['file'], $error['line']);
}

// Custom error data (e.g. Validations)
if (method_exists($e, 'getData')) {
$errors = $e->getData();
}

if (!empty($errors)) {
$error['errors'] = $errors;
}

$log->error($e->getMessage());
if ('application/json' === $mediaType || true === $isAPI) {
$app->response->headers->set(
'Content-Type',
'application/json'
);
echo json_encode($error);
} else {
echo '<html>
<head><title>Error</title></head>
<body><h1>Error: ' . $error['code'] . '</h1><p>'
. $error['message']
.'</p></body></html>';
}

});

The $app->error() method receives the thrown exception as argument. By default I fetch all the data I need and fill the $error array, then if I’m in production mode I unset the private data and rewrite the message with a generic one. The custom ValidationException class has a custom getData() method that returns an array of validation errors that are added to the final payload. Then the error is displayed in JSON or HTML depending on the request. On the API side we can have a simple error like this:

{
"code": 123,
"message": "An error occurred, a support person is being notified of this"
}

or a full validation error like this:

{
"code": 0,
"message": "Invalid data",
"errors":
{
"contact":
[
{
"field": "email",
"message": "Email address already exists"
}
]
}
}

Conclusion

We have the core of our API in place now. In the next part we’ll add some flesh in order to have a full functioning service. In the meanwhile, feel free to read through the articles linked throughout this part – they’re a treasure trove of useful API design principles.

Frequently Asked Questions (FAQs) about Building a REST API from Scratch