$bugun= date('Y-m-d');
$birincidonembaslangic= date('Y-m-d', '2019-08-01');
$birincidonembitis= date('Y-m-d', '2019-01-31');

if ($bugun < $birincidonembaslangic && $bugun > $birincidonembitis)
{
echo "1.Dönemdeyiz";
}

$ikincidonembaslangic= date('Y-m-d', '2019-02-01');
$ikincidonembitis= date('Y-m-d', '2019-07-31');

if ($bugun < $ikincidonembaslangic && $bugun > $ikincidonembitis)
{
echo "2.Dönemdeyiz";
}