You seem to be mixing your single and double quotes. You should decide which ones you are going to use to quote strings and then use that one consistently. I believe single-quotes are slightly faster as PHP doesn't parse them but the performance gain will only be very very slight and you might decide the convenience of double-quotes is better.
So for your example, with double-quotes:
echo "<a href='?cal_start=$next&property_id={$_GET['property_id']}'>></a>";
and, with single quotes
echo '<a href="?cal_start='.$next.'&property_id='.$_GET['property_id'].'">></a>';