$paymentDate = date('Y-m-d');
$paymentDate=date('Y-m-d', strtotime($paymentDate));
//echo $paymentDate; // echos today!
$contractDateBegin = date('Y-m-d', strtotime("01/01/2001"));
$contractDateEnd = date('Y-m-d', strtotime("01/01/2012"));
if (($paymentDate >= $contractDateBegin) && ($paymentDate <= $contractDateEnd)){
echo "is between";
}else{
echo "NO GO!";
}