Get Time From 5 Minutes Prior - php

Question

I'm working on User Functions for the CMS I'm working on and I have users updated on each page load Using
if(User("online") == true) {
LastOnlineUpdate($_SESSION['key']['userid']);
}
(Yes User Online Works, That's my function to check online users)
And now my times are updated correctly hooking into the LastOnlineUpdate function then running my class function
public function SessionUpdate($arg) {
$query = <<<SQL
UPDATE {$this->tprefix}online
SET remote = :remote, timestamp = :timestamp
WHERE userid = :arg
SQL;
$resource = $this->db->db->prepare( $query );
$resource->execute( array(
':remote' => $_SERVER['REMOTE_ADDR'],
':timestamp' => time(),
':arg' => $arg,
));
}
Now whenever I go through my pages it updates the user by having the first code within my preloaded file, now I've hit my roadblock though with something I've never done and that's to take the time that's been inserted into my table, select it if the time is less than five minutes and utilize it for my online users section
public function OnlineUsers() {
$query = <<<SQL
SELECT userid
FROM {$this->tprefix}online
WHERE timestamp > time(5Minutes)
SQL;
$resource = $this->db->db->prepare( $query );
$resource->execute();
$this->onlinecount = $resource->rowCount();
if($resource->rowCount() == 0 ) {
$this->onlineusers = "No User's Online";
}
else
{
foreach($resource as $row) {
self::ConvertIDToName("displayname"); //Can be display, user, first or whatever else for the argument, The onlineusers variable is then set based off that query
}
}
}

Answer 1

$minutes = 5;
$date = gmdate('Y-m-d H:i:s'); // http://php.net/manual/en/function.gmdate.php
$date_plus_minutes = gmdate('Y-m-d H:i:s', ( time() + $minutes * 60 ) );
$date_minus_minutes = gmdate('Y-m-d H:i:s', ( time() - $minutes * 60 ) );