private static function commandExists($command) {
		$whereIsCommand = (PHP_OS == 'WINNT') ? 'where' : 'which';
		$process = proc_open("$whereIsCommand $command", [
			0 => array("pipe", "r"), //STDIN
			1 => array("pipe", "w"), //STDOUT
			2 => array("pipe", "w"), //STDERR
				], $pipes);

		if ($process !== false) {
			$stdout = stream_get_contents($pipes[1]);
			$stderr = stream_get_contents($pipes[2]);
			fclose($pipes[1]);
			fclose($pipes[2]);
			proc_close($process);

			return $stdout != '';
		}

		return false;
	}