Found the solution… the problem was due to placement of the conditional statement in the wp-config.php
file.
It needed to be inserted before the require_once(ABSPATH . 'wp-settings.php');
line near the bottom of the default config file, otherwise the constants are already defined and cannot be redefined.
So final section of the wp-config.php
file now looks like this:
...
/** Add xip.io support for mobile theme testing **/
require_once('b5f-browsers.php');
if( b5f_browser_check('mobile')) {
define('WP_HOME','http://mysite.dev.'.$_SERVER['SERVER_ADDR'].'.xip.io');
define('WP_SITEURL','http://mysite.dev.'.$_SERVER['SERVER_ADDR'].'.xip.io');
}
/* That's all, stop editing! Happy blogging. */
/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
define('ABSPATH', dirname(__FILE__) . "https://wordpress.stackexchange.com/");
/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');