I had the same problem and found, that function is_executable() will be from PHP 5 version for windows. As a result, why don't you use function function_exists? next code for robot_functions.php line ~674:
PHP Code:
if (USE_IS_EXECUTABLE_COMMAND == 1 && function_exists("is_executable")) {
$is_exec_command_msword = is_executable(PHPDIG_PARSE_MSWORD);
$is_exec_command_msexcel = is_executable(PHPDIG_PARSE_MSEXCEL);
$is_exec_command_pdf = is_executable(PHPDIG_PARSE_PDF);
}
else {
$is_exec_command_msword = 1;
$is_exec_command_msexcel = 1;
$is_exec_command_pdf = 1;
}