Twitter has been used in such a function to generate a pretty date format string.

pretty_date.php

<?php
function pretty_date($time) {
	$diff = time() - $time;
	$day_diff = floor($diff / 86400);
	
	if(is_nan($day_diff)) return '';

	if ($day_diff == 0) {
		if ($diff < 60) {
			return $diff . "second ago";
		} else if ($diff < 120) {
			return '1 min ago';
		} else if ($diff < 3600) {
			return floor( $diff / 60 ) . " min ago";
		} else if ($diff < 7200) {
			return '1 hour ago';
		} else if ($diff < 86400) {
			return floor( $diff / 3600 ) . " hour ago";
		}
	} else if ($day_diff == 1) {
		return 'yesterday';
	} else if ($day_diff < 7) {
		return $day_diff . " day ago";
	} else if ($day_diff < 31) {
		return ceil( $day_diff / 7 ) . " week ago";
	} else {
		return '';
	}
}

Thanks!