In fact in the short run the only way to vary output is by varying the amount of the variable input.

Conventionally the variable input is assumed to be labor.

The key factor is that the variable input is being changed while all other factors of production are being held constant.

Diminishing returns occur when the marginal product of the variable input is negative.

That is when a unit increase in the variable input causes total product to fall.

That is, what is the profit maximizing usage of the variable input?

In the short run, increasing production requires using more of the variable input - conventionally assumed to be labor.

Their formula used 5 variable inputs and 6 constants.

The output per unit of both the fixed and the variable input declines throughout this stage.

Another way to achieve that is to use a prepared statement to sanitize all variable input for a query.