function time_elapsed($secs){
    $bit = array(
        'y' => $secs / 31556926 % 12,
        'w' => $secs / 604800 % 52,
        'd' => $secs / 86400 % 7,
        'h' => $secs / 3600 % 24,
        'm' => $secs / 60 % 60,
        's' => $secs % 60
        );

    foreach($bit as $k => $v)
        if($v > 0)$ret[] = $v . $k;

    return join(' ', $ret);
}

//Tempo inicial
$ini = time();

//Tarefa exemplo
sleep(5);

//Tempo final
$out = time();

echo time_elapsed($out-$ini);