$month = 3;
$year = 2017;
$fridaysOfMonth = []; // массив с пятницами месяца
//
$daysInMon = cal_days_in_month(CAL_GREGORIAN, $month, $year);
foreach(range(1,$daysInMon) as $day) {
    $dayOfWeek = date('N', strtotime( sprintf('%02d.%02d.%d',$day,$month,$year) ) );
    if ($dayOfWeek == 5) {
        $fridaysOfMonth[] = $day;
    }
}//foreach
$countOfFridays = sizeof( $fridaysOfMonth ); // количество пятниц в месяце
echo sprintf('В месяце %02d.%d пятниц: %d', $month, $year, $countOfFridays);