strtotime() reads the timestamp in en_US format if you want to change the date format with this number, you should previously know the format of the date you are trying to parse. Let's say you want to do this :
strftime("%Y-%m-%d",strtotime("05/11/2007"));
It will understand the date as 11th of may 2007, and not 5th of november 2007. In this case I would use:
$date = explode("/","05/11/2007");
strftime("%Y-%m-%d",mktime(0,0,0,$date[1],$date[0],$date[2]));
Much reliable but you must know the date format before. You can use javascript to mask the date field and, if you have a calendar in your page, everything is done.
Thank you.