What is default value in PHP

I have a problem with default value for $_POST[]; So i have a html form with textboxes and the informations is sent to a php script. The php script has a sql query that is being sent to my database. But if my textbox in html form is empty the query doesnt have a value. So i want to set my post to a default value 0 so it returns a value atleast.

So here is an example of html form (This is not my actuall script. Just an example.

Ok so this script will send both id and test textboxes will always have a number value. And it sends the information to testscript.php

Here is testscript.php example

Alright so now if i submit my html form to php script without inserting any text to the textboxes the query will look like this

But the query should be like this

So. I know i can use value attribute in the html tag but then the value will be visible in the textbox and i dont want that.

And i know i can do an if statment to make a default value like this

But now the problem is that i would have to do that if statment for every textbox and my html form have more than 100 textboxes. So i dont want to make an if statment for every textbox because then my script will be way to big and it will take hours.

So is there any way to set a default value for all the textboxes without using if statment in php or value attribute in html form?