/**
 * Custom JSON callback.
 */
add_action( 'init', function () {
    // Get URI without query string.
    $uri = strtok( $_SERVER["REQUEST_URI"] , '?' );

    // Check callback path.
    $reg = preg_match( '/\/json-callback\/?$/', $uri, $matches );
    if ( count( $matches ) > 0) {
        // Set header.
        status_header(200);

        // Output JSON of parameters.
        wp_send_json($_GET);

        exit;
    }
} );