note 82480 added to function.sha1

How do you undo a SHA1(), I've used this function to "encrypt" a password, but let's say the user forget's their password, if I e-mail it to them then they will get a weird set of numbers that will not work when they want to log in, so how do I reverse a SHA1() function, or is it possible? If not could someone please tell me an alternative to this, something simple please I'm only thirteen and programming a huge website. ---- Server IP: 216.235.15.211 Probable Submitter: 24.141.143.234 ---- Manual Page -- http://www.php.net/manual/en/function.sha1.php Edit -- https://master.php.net/note/edit/82480 Del: integrated -- https://master.php.net/note/delete/82480/integrated Del: useless -- https://master.php.net/note/delete/82480/useless Del: bad code -- https://master.php.net/note/delete/82480/bad+code Del: spam -- https://master.php.net/note/delete/82480/spam Del: non-english -- https://master.php.net/note/delete/82480/non-english Del: in docs -- https://master.php.net/note/delete/82480/in+docs Del: other reasons-- https://master.php.net/note/delete/82480 Reject -- https://master.php.net/note/reject/82480 Search -- https://master.php.net/manage/user-notes.php