strtotime("") returns the timestamp for midnight on the current date.

I suppose if you think about it a bit, it makes sense (since the modification you are making to the current date is empty), but I would suggest that it should return either a warning or nothing.

A lot of people (judging from the manual notes) use strtotime() to parse an existing date string, not to do a "+30 days" modification to it.  For them, the strtotime("") result is not intuitive, I think.

Perhaps the confusion could be fixed by having one function to parse date/time strings and one to do the "+X days" modification?