i'm not quite sure if php automatically converts a null into sql NULL, if not then that could be a problem..
try
mysql_query("INSERT INTO ".$_POST['table']." (".$_POST['c1'].", ".$_POST['c2'].", ".$_POST['c3'].", ".$_POST['c4'].", ".$_POST['c5'].", ".$_POST['c6'].", ".$_POST['c7']."...