This is a bit of a roundabout way of doing it but it should work. How long is a lunar month? let's say 20 days. You have the date for a full moon let's say 23/08/2003 now you can do.
<?
if(round((mktime(0,0,0,8,23,2003)/(24*60*60))-(time()/(24*60*60)))%20==0)
{
echo("tonight will be a full moon");
}
else
{
echo("tonight will not be a full moon");
}
?>
HTH
Rob