It would be nice to have some sort of method that works like this:

$ttl = phutil_units('30 days in seconds');

It would parse the string, throw if it couldn't interpret it, and otherwise compute the desired value. This would make this kind of junk:

$ttl = (60 * 60 * 24 * 30); // 30 days

...self-documenting and impossible to get wrong. In almost every case where we use a constant like this, I suspect we can spare the extra function call and regexp.