Login to your
Account Panel and scroll down to "PHP Version". Apply for an upgrade to PHP Version 2, citing BTI Tracker breaking as a reason. ATM you're on the default of PHP Version 1, which disables almost every PHP function in the name of security. Version 2 is enough for most websites, as it enables all the functions you really need; users will only be upgraded to Version 3 if they genuinely need it and have a good reason for it, because V3 enables nearly every PHP function under the sun, except a couple of really nasty ones.