ProTip: Test the key exists before accessing the value in the switch.

test-before-observing.php

<?php

switch (isset($headers['content-type']) && $headers['content-type']) {
    case 'application/xml':
        $format = 'XML';
        break;
    case 'application/json':
    default:
        $format = 'JSON';
}
echo $format;