A PHP script for converting between different datetimes

This script converts a Unix timestamp (Seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)) to the International Standard ISO 8601 date and time notation (YYYY-MM-DD hh:mm:ss) and vice versa. I find it useful for verifying datetimes.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN”
“http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
<html xmlns=”http://www.w3.org/1999/xhtml” xml:lang=”en” lang=”en”>
<head>
<meta http-equiv=”Content-Type” content=”text/html; charset=iso-8859-1″ [...]

Protecting a download using a unique URL

A client asked me to develop a simple method for protecting a download (or digital product) by generating a unique URL that can be distributed to authorized users via email. The URL would contain a key that would be valid for a certain amount of time and number of downloads. The key will [...]