datetime_elapsed.php

<?php
$datetime = '2015-12-07 00:58:18';
$now = new DateTime;
$ago = new DateTime($datetime);
$diff = $now->diff($ago);

$diff->w = floor($diff->d / 7);
$diff->d -= $diff->w * 7;

$string = [
    'y' => 'year',
    'm' => 'month',
    'w' => 'week',
    'd' => 'day',
    'h' => 'hour',
    'i' => 'minute',
    's' => 'second',
];
foreach ($string as $k => &$v) {
    if ($diff->$k) {
        $v = $diff->$k . ' ' . $v . ($diff->$k > 1 ? 's' : '');
    } else {
        unset($string[$k]);
    }
}

if (!$full) $string = array_slice($string, 0, 1);

return $string ? implode(', ', $string) . ' ago' : 'just now';