<"MY_CONSTANT", 1);
print_r(get_defined_constants(true));
?> 
The above example will output something similar to:
Array
(
  [internal] => Array
    (
      [E_ERROR] => 1
      [E_WARNING] => 2
      [E_PARSE] => 4
      [E_NOTICE] => 8
      [E_CORE_ERROR] => 16
      [E_CORE_WARNING] => 32
      [E_COMPILE_ERROR] => 64
      [E_COMPILE_WARNING] => 128
      [E_USER_ERROR] => 256
      [E_USER_WARNING] => 512
      [E_USER_NOTICE] => 1024
      [E_ALL] => 2047
      [TRUE] => 1
    )
  [pcre] => Array
    (
      [PREG_PATTERN_ORDER] => 1
      [PREG_SET_ORDER] => 2
      [PREG_OFFSET_CAPTURE] => 256
      [PREG_SPLIT_NO_EMPTY] => 1
      [PREG_SPLIT_DELIM_CAPTURE] => 2
      [PREG_SPLIT_OFFSET_CAPTURE] => 4
      [PREG_GREP_INVERT] => 1
    )
  [user] => Array
    (
      [MY_CONSTANT] => 1
    )
)