I think the problem is that your option elements do not have a value attribute. Option elements have two major elements:
* the value attribute, which is what will be the select's value and what will be sent to the server.
* the text (between the start and end tags), which is what you see on screen.
Since you have no value attributes, the value remains empty when sent to insert.php.
And can you please use code tags next time? I've added them for you this time.
I also noticed that your select element has no name. Just like input elements, select elements need a name to be sent to the server. You can then retrieve the value of the selected option from the $_POST array.
1) It's $_POST, not $_post. PHP is case sensitive.
2) You use $combo.=. The .= means that you want to append to an existing variable. Since this is the first occurrence of $comboyou should drop the .: $combo=$_POST["combo"];