The current day of the week can be found with date('w');. Sunday of the current week was that many days ago. Saturday is six days after that, and Sunday of next week is the day after Saturday.
// Doesn't matter if $sunday to be zero or negative
$sunday = date('d')-date('w');
$month = date('m');
$year = date('Y');
// In fact it doesn't matter that mktime() here will be asked to calculate
// the timestamp for Saturday the 83rd of December 2008. It's
// smart enough to figure out that that's the 21st of February 2009.
for($i=0; $i<7; $i++)
{
$weeks[$i]['Sun'] = mktime(0,0,0, $month, $sunday+7*$i, $year);
$weeks[$i]['Sat'] = mktime(0,0,0, $month, $sunday+7*$i+6, $year);
}