function get_week($year) { 
$year_start = $year . "-01-01"; 
$year_end = $year . "-12-31"; 
$startday = strtotime($year_start); 
if (intval(date('N', $startday)) != '1') { 
$startday=strtotime("nextmonday",strtotime($year_start));
//获取年第一周的日期 
} 
$year_mondy = date("Y-m-d", $startday);//获取年第一周的日期 
$endday = strtotime($year_end); 
if (intval(date('W', $endday)) == '7') { 
$endday=strtotime("lastsunday",strtotime($year_end)); 
} 
$num = intval(date('W', $endday)); 
for ($i = 1; $i <= $num; $i++) { 
$j = $i -1; 
$start_date = date("Y-m-d", strtotime("$year_mondy $j week ")); 
$end_day = date("Y-m-d", strtotime("$start_date +6 day")); 
$week_array[$i] = array( 
str_replace("-",".",$start_date),str_replace("-",".",$end_day)); 
} 
return $week_array; 
}

$weeks = get_week(2011); 
echo '第18周开始日期：'.$weeks[18][0].''; 
echo '第18周结束日期：'.$weeks[18][1];

1. 第18周开始日期：2011.05.02 
2. 第18周结束日期：2011.05.08