php计算多长时间之前的函数

给定一个时间，计算这个时间在多久前，比如：2天前，1年前

<?php 
function prettyDate($date){ 
    $time = strtotime($date); 
    $now = time(); 
    $ago = $now - $time; 
    if($ago < 60){ 
        $when = round($ago); 
        $s = ($when == 1)?"second":"seconds"; 
        return "$when $s ago"; 
    }elseif($ago < 3600){ 
        $when = round($ago / 60); 
        $m = ($when == 1)?"minute":"minutes"; 
        return "$when $m ago"; 
    }elseif($ago >= 3600 && $ago < 86400){ 
        $when = round($ago / 60 / 60); 
        $h = ($when == 1)?"hour":"hours"; 
        return "$when $h ago"; 
    }elseif($ago >= 86400 && $ago < 2629743.83){ 
        $when = round($ago / 60 / 60 / 24); 
        $d = ($when == 1)?"day":"days"; 
        return "$when $d ago"; 
    }elseif($ago >= 2629743.83 && $ago < 31556926){ 
        $when = round($ago / 60 / 60 / 24 / 30.4375); 
        $m = ($when == 1)?"month":"months"; 
        return "$when $m ago"; 
    }else{ 
        $when = round($ago / 60 / 60 / 24 / 365); 
        $y = ($when == 1)?"year":"years"; 
        return "$when $y ago"; 
    } 
} 
echo prettyDate("2012-07-22 12:23:45")."<br />"; 
echo prettyDate("2010-11-12 22:25:45")."<br />"; 
echo prettyDate("2012-01-01 01:00:00")."<br />"; 
echo prettyDate("2001-05-30 00:00:00")."<br />";