What could be causing the sudden incorrect display of special characters in a mobile app when fetching data from a WordPress site as JSON?

The sudden incorrect display of special characters in a mobile app when fetching data from a WordPress site as JSON could be due to encoding issues. To solve this problem, you can use the `utf8_encode()` function in PHP to encode the data before returning it as JSON.

// Fetch data from WordPress site
$data = get_data_from_wordpress();

// Encode data using utf8_encode
$data_encoded = utf8_encode($data);

// Return encoded data as JSON
header('Content-Type: application/json');
echo json_encode($data_encoded);