get_loaded_extensions

The above example will output something similar to:

Array
(
   [0] => xml
   [1] => wddx
   [2] => standard
   [3] => session
   [4] => posix
   [5] => pgsql
   [6] => pcre
   [7] => gd
   [8] => ftp
   [9] => db
   [10] => calendar
   [11] => bcmath
)