$a = $_POST['calc'];
$b = $_POST['kof'];
$c = $_POST['tar'];
$c =(double)$c;
$b = (double)$b;
$mon = $b*$c;
$mon = (string)$mon;
$b = (string)$b;
$myproject = mysql_connect('localhost', 'root', '') or die ('Error in connecting');
mysql_select_db("oplata", $myproject) or die ('error in selecting DB');
echo $a;
$sql1="INSERT INTO $a (data,zachet,tenge) VALUES ('$date','$b','$mon')";
mysql_query($sql1,$myproject) or die (mysql_error());
И пищет ошибку
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(data,zachet,tenge) VALUES ('25.02.2014','0.5','400')'