$weekday = date("w");
$stamp = strtotime(date("Y-m-d"));
if($weekday > 0){
$weekstartdate = date("Y-m-d", $stamp - ( 86400 * $weekday) );
}else{
$weekstartdate = date("Y-m-d", $stamp);
}
echo $weekstartdate;
This gives the date of the Sunday week start for the current week. Monday requires a little tweaking (will leave that up to you 🙂 )