Outline the process of integrating a third-party API into a WordPress website, specifying methods for secure data handling and interaction.

Integrating a third-party API into a WordPress website allows you to extend the functionality of your site by utilizing external services. This process involves several crucial steps, including making API calls, handling data securely, and displaying information appropriately. Here's a detailed breakdown of the integration process with emphasis on security and usability:

1. Understand the API Documentation:
Before starting the integration, thoroughly review the API documentation provided by the third-party service. This documentation will explain the various endpoints, request methods (GET, POST, PUT, DELETE, etc.), required parameters, authentication methods, and response formats (usually JSON or XML). Understanding this is crucial for proper integration. It's imperative to understand the API limits, rate limiting, and authentication procedures of the third party API.

2. Choose an Appropriate Method for Making API Requests:
WordPress doesn't have a built-in function for making external API calls. The most common methods to make API requests are the WordPress `wp_remote_get()`, `wp_remote_post()` and other related functions. These functions make HTTP requests and process the response using server-side code. Avoid making API requests directly from the client-side (JavaScript) because that would expose sensitive information like API keys. WordPress also has the `WP_Http` class if you need more control of the requests.

Example using `wp_remote_get()`:
```php
function get_external_api_data() {
$api_url = 'https://api.example.com/data';
$api_key = get_option('my_plugin_api_key'); // Retrieve securely stored API key
$headers = array('Authorization' => 'Bearer ' . $api_key); // Authorization header

$response = wp_remote_get( $api_url, array('headers' => $headers) );

if ( is_wp_error( $response ) ) {
$error_message = $response->get_error_message();
error_log( 'API Request Failed: ' . $error_message ); // Log the error
return null;
}

$body = wp_remote_retrieve_body( $response );
$data = json_decode( $body, true ); // Decode the JSON response

if ( is_array( $data ) && ! empty( $data ) ) {
return $data;
}
else {
error_log( 'Failed to retrieve data from the API.' );
return null;
}
}
```
This code makes an API request with `wp_remote_get()`, adds an authorization header, and then returns the decoded JSON data if successful, while logging errors if there is a problem.

3. Store API Keys and Sensitive Data Securely:
Never hardcode API keys or other sensitive information directly into the plugin or theme files. Instead store them securely using the WordPress Options API. This involves using functions like `add_option()`, `update_option()`, and `get_option()`. This ensures that keys are stored encrypted in the database. Always perform API calls from server side. Avoid making API requests from client side code as the API keys will be exposed to users.

Example storing and retrieving API keys:
```php
// Add an API key in the settings page of a plugin
add_option('my_plugin_api_key', 'YOUR_API_KEY_HERE', '', 'yes');

// Function to retrieve the API key.
function get_my_plugin_api_key() {
return get_option('my_plugin_api_key');
}
```
This example shows how you can retrieve and store an api key securely using WordPress settings API.

4. Handle API Responses Properly:
API responses are typically in JSON or XML format. Use PHP functions like `json_decode()` or `simplexml_load_string()` to parse the response data. Check for errors in the response, and log any issues for troubleshooting. Ensure that the data you receive from the API is in the expected format before displaying or processing it further.

Example to handle a JSON response:
```php
$api_response = get_external_api_data(); // Function from above

if ( ! empty( $api_response ) ) {
foreach( $api_response as $item ) {
// Process each item here
echo '<p> Title: ' . esc_html( $item['title'] ) . '</p>';
echo '<p> Content: ' . esc_html( $item['content'] ) . '</p>';
}
} else {
echo 'Failed to display data.';
}
```
This snippet loops through the JSON data and display the data safely after escaping it.

5. Implement Caching to Reduce API Calls:
To avoid overwhelming the third-party API and to improve your website performance, implement caching. You can use WordPress transients or the WordPress object cache to store API responses temporarily. If the response is cached, serve the cached response rather than making the API request. Clear the cache if the data is updated. The transient expires after a certain duration and is removed from the database.

Example of caching using transients:
```php
function get_cached_api_data() {
$transient_key = 'my_plugin_api_data';
$cached_data = get_transient( $transient_key );

if ( false === $cached_data ) {
$data = get_external_api_data(); // Make the API call
if ( ! empty ( $data ) ) {
set_transient( $transient_key, $data, 1 HOUR_IN_SECONDS ); // Save cached data for 1 hour
return $data;
} else {
return null; // Return if unable to retrieve data
}

}
return $cached_data; // Return cached data if exists
}
```
This code stores the result of the API call for one hour to reduce API requests and to speed up the website loading time.

6. Sanitize and Validate Data:
Always sanitize and validate any data received from the API before displaying it on your website. This prevents XSS attacks or any other security vulnerabilities. Use functions like `esc_html()`, `esc_attr()`, `wp_kses_post()` and sanitize functions provided by WordPress.

7. Handle Errors and Exceptions:
Implement error handling to gracefully handle situations where the API request fails. Log any errors or warnings and display user-friendly messages instead of exposing technical details to the user. This can be done by using `is_wp_error` and `error_log` functions provided by wordpress.

8. Follow API Rate Limits:
Respect the third-party API's usage limits. If you exceed the rate limits, your IP address might get temporarily banned. Implement rate limiting logic to ensure you are not overwhelming the external API server.

9. Use HTTPS:
Ensure both your WordPress website and the third-party API use HTTPS to encrypt data and secure the connection. Use HTTPS for all API requests.

By following these steps, you can securely and effectively integrate a third-party API into your WordPress website, providing enhanced functionality while maintaining security and user experience. Always prioritize security by never exposing API keys on the client side code, and always sanitize and validate all input and output.