Chapter 2: Extending Functionality with Custom REST API Endpoints and External Data

Overview

In this chapter, we extend your WordPress functionality by creating a custom REST API endpoint using pure PHP. This endpoint will fetch live data from Coinbase’s currencies API, process it, and return a custom JSON structure. You’ll also learn how to test and use this endpoint in your WordPress site.

Key Topics

• Registering a custom REST API endpoint using rest_api_init.
• Writing a callback function that fetches data via wp_remote_get() and processes the JSON response.
• Exposing a tailored JSON output that can be consumed by external applications or your own custom templates.

Demo: Create a Custom REST API Endpoint for Coinbase Currencies

Add the following code to your theme’s functions.php file (or in a custom plugin) to register an endpoint that fetches and returns live currency data from Coinbase.

<?php
// Register a custom REST API endpoint.
add_action('rest_api_init', function () {
    register_rest_route('custom/v1', '/coinbase-currencies', array(
        'methods'  => 'GET',
        'callback' => 'custom_get_coinbase_currencies',
    ));
});

function custom_get_coinbase_currencies() {
    // Fetch data from the Coinbase currencies API.
    $response = wp_remote_get('https://api.coinbase.com/v2/currencies');
    
    // Check for errors in the API response.
    if ( is_wp_error( $response ) ) {
        return new WP_Error('api_error', 'Error fetching data from Coinbase API', array('status' => 500));
    }
    
    // Retrieve and decode the API response.
    $body = wp_remote_retrieve_body( $response );
    $json = json_decode( $body, true );
    
    // Process the data: limit to the first 10 currencies for this demo.
    $currencies = ! empty( $json['data'] ) && is_array( $json['data'] ) 
        ? array_slice( $json['data'], 0, 10 ) 
        : array();
    
    // Return a custom JSON structure.
    return array(
        'source'     => 'Coinbase',
        'currencies' => $currencies
    );
}
?>

Testing the Endpoint

After adding the code, test your new REST API endpoint by visiting the following URL in your browser:

https://yourdomain.com/wp-json/custom/v1/coinbase-currencies

You should see a JSON response similar to:

{
    "source": "Coinbase",
    "currencies": [
        {
            "id": "AED",
            "name": "United Arab Emirates Dirham",
            "min_size": "0.01"
        },
        {
            "id": "AFN",
            "name": "Afghan Afghani",
            "min_size": "0.01"
        },
        // ... more currencies ...
    ]
}

Explanation of the Code

• Endpoint Registration:

  The rest_api_init action hook is used to register a new endpoint under the namespace custom/v1 with the route /coinbase-currencies.

• Callback Function:

  The custom_get_coinbase_currencies function handles the GET request. It uses wp_remote_get() to fetch data from the Coinbase API, checks for errors, decodes the JSON response, and limits the result to the first 10 currencies.

• Custom JSON Structure:

  The endpoint returns an associative array that includes the source of the data and a list of currencies. WordPress automatically converts this array to JSON when the endpoint is accessed.

Why Use Custom REST API Endpoints?

Custom REST API endpoints extend the functionality of your WordPress site. They enable you to:

• Integrate live external data seamlessly.
• Expose custom data structures to external applications.
• Create a more dynamic and interactive website experience.

Conclusion

In this chapter, you learned how to create a custom REST API endpoint in WordPress using pure PHP. You saw how to fetch and process live data from Coinbase’s currencies API and return it in a custom JSON format. This powerful technique allows you to integrate external data sources and build interactive, data-driven applications.

In the next chapter, we will focus on best practices, security, and advanced customizations to further enhance your WordPress and PHP integrations.