Importancy algorithm based on time - php

Question

I have an array of users, as follows;
<?php
$users = array(
array(
"id" => 1,
"last_updated" => 1398933140,
"weight" => 2.0
),
array(
"id" => 2,
"last_updated" => 1398933130,
"weight" => 0
),
array(
"id" => 3,
"last_updated" => 1398933120,
"weight" => 1.0
),
array(
"id" => 4,
"last_updated" => 1398933110,
"weight" => 0
)
);
?>
I want to (asynchronously) refresh some stats on users (for which I'm using a crobjob) ordered on last_updated, in essence the user with the most stale stats.
However, I want to add weights to users and calculate that into the equation. Is it good enough to just convert the weight to some amount of seconds and substract that from the last_updated timestamp?
I know my example array has time differences of 10 seconds, but I only want to start adding the weight criteria after 3600 seconds.
// Start substracting weight
if ($timediff > 3600) {
// The longer the timediff, the heavier the weight becomes
$weight_severity = (($timediff/1000) * $weight) * SOME_CONSTANT;
// Putting the 'last_updated' back farther in the past
$substract = $timestamp - $weight_severity;
}
Is this a good 'algorithm' or will this go horribly wrong when the differences become pretty large?
At the moment I have nearly 2000 users (expected will become 10.000), so theoretically a full loop takes 2000 minutes. My concern is, will a user with a weight of 2.0 be buried under 500 'insignificant' users?
Update: I have enhanced my code a bit.
<?php
$users = array(
array(
"id" => 1,
"last_updated" => 1399281955,
"weight" => 2.0
),
array(
"id" => 2,
"last_updated" => 1399281955 - 15000,
"weight" => 0
),
array(
"id" => 3,
"last_updated" => 1399281955 - 30000,
"weight" => 1.0
),
array(
"id" => 4,
"last_updated" => 1399281955 - 45000,
"weight" => 0
)
);
$results = array();
foreach ($users as $index => $user) {
$factor = 3;
$timestamp = $user['last_updated'];
$substract = $timestamp;
// Start substracting weight
$timediff = time() - $timestamp;
if ($timediff > 3600) {
// The longer the timediff, the heavier the weight becomes
$weight_severity = pow((($timediff/1000) * $user['weight']), $factor);
// Putting the 'last_updated' back farther in the past
$substract = $timestamp - $weight_severity;
}
$users[$index]['weight_updated'] = floor($substract);
$users[$index]['timediff'] = $timediff;
$users[$index]['diff'] = $users[$index]['last_updated'] -
$users[$index]['weight_updated'];
}
echo '<pre>';
print_r($users);
usort($users, function($a, $b) {
return (($a['weight_updated'] == $b['weight_updated'])) ?
0 : ($a['weight_updated'] < $b['weight_updated']) ? -1 : 1;
});
print_r($users);
So without weights, the user IDs would be: 4,3,2,1. But with my 'algorithm' it's now 3,4,2,1. User ID 3 because of it's weight, is getting done before 4. This is with a time difference of 15000 seconds (a little over 4 hours).