checking for utilities/applications in the PATH

hi, i am trying to do a simple check for archiving utilities (zip, unzip, ...), to make sure they exist within the PATH, and are executable by PHP. i could not find a better way to do this through php, so i went ahead and did an exec('which ' . $utility, $output, $return_value); then, i check the $return_value for a non-zero value, which would indicate that the utility was not found. using GNU/Linux this seems to work fine, but i wouldn't mind keeping my work cross-platform. so, my question is, is there a better way to do this? and, if not, what i have done, for now is the following: $result = exec('which ' . $utility, $output, $return_val); return( $result && !$return_val ); from one quick test i ran, i determined that exec() simply returns NULL or nothing if the given utility/command doesn't exist (which, in this case). so, i expect the above code to simply return true, or skip the test for my $utility rather, if the which utility is not present on the system. will this work as expected in all cases/systems? thanks a lot.