Best approach to manage backend input validation

I am making a web application, and I need to sanitize and validate input data (e.g. data passed to a REST service).

I have already read that input validation belongs to the model, in the MVC paradigm.

I am now wondering what's the best DRY approach to do this. Currently my code looks pretty much like the following:

    class myModel
    {
        public static foo($input1, $input2)
        {
            /*INPUT1 VALIDATION*/
            /*...*/
            /*INPUT2 VALIDATION*/
            /*...*/

            /*IF AT THIS POINT THE METHOD HAS NOT RETURNED YET, PROCEED WITH THE ACTUAL CODE*/
            /*...*/
        }
    }

In this way, though, validation is repeated multiple times, once for each method call of the model.

Does a better approach exist?