<?php // nice and dirty code
mysql_connect("localhost", "dbuser", "dbpass") or die(mysql_error()); // connect to database
mysql_select_db("dbname") or die(mysql_error()); // select database
$timestamp = '1286344241';
$page_query = mysql_query("SELECT * FROM xf_user WHERE register_date = $timestamp") or die(mysql_error());
// using while in case we wanna do more stuff or have to handle more results (unlikely since we're dealing with timstamps though)
while ($row = mysql_fetch_assoc($page_query)) {
$username = $row['username'];
}
$datetime = date("Y-m-d H:i:s", $timestamp);
echo "timestamp $timestamp = date: $datetime and it seems that it's $username";