$date_start="2011-07-01"; # начальная дата
$date_finish="2011-08-10";# конечная дата
echo '<pre>';

echo "\ndate_start = ".$date_start;
echo "\ndate_finish = ".$date_finish;
$date_time_start    =strtotime($date_start);
$date_time_finish   =strtotime($date_finish);

$WeekNumStart = date('W',$date_time_start);
$WeekNumEnd = date('W',$date_time_finish);
echo "
WeekNumStart = $WeekNumStart
WeekNumEnd  = $WeekNumEnd";

$WeekDayStart = date('w',$date_time_start);
$WeekDayEnd = date('w',$date_time_finish);

echo "
WeekDayStart = $WeekDayStart
WeekDayEnd = $WeekDayEnd
";

echo '
*****************************************
проходим день за днем.
';
$daysec = 86400;
//сетчик кол-ва понедельников, вторников и т.д.
$WeekCounter = array(
        0=>0,   //восскрессенье
        1=>0,   // понедельник
        2=>0,   //вт
        3=>0,   //ср
        4=>0,
        5=>0,
        6=>0,   //сб
);
for($d=$date_time_start; $d<$date_time_finish; $d+=$daysec)
{
    $WeekCounter[(int)date('w',$d)]++;
}

print_r($WeekCounter);