Validate file if a select is...

i , sorry for english , i want validate a form with a  select and file , only if select  value is 6 the file must be uploaded .

How can i create this rule  ?

this is my code:

Code:

 <select id='id_id_stato_pratica'  class="form-control border border-warning mxn-shadow-input mb-4" name='id_stato_pratiche' >
                                  
                                  
       <option value='4'> FIRMATA </option>

       <option value='5'> IN LAVORAZIONE </option>
                                
       <option value='6'>  COMPLETATA </option>

</select>

<input type="file" name="pratica" id='file_pratica'>

   

This is my validation but don't work :

Code:

    public function validate_pratica_completata(string $str,string $fields, array $data)
    {

        if($data['id_stato_pratiche'] == 5){

            return true;

        }elseif ($data['id_stato_pratiche'] == 6 AND isset($data['pratica'])) {

            return true;

        }else{

            return false;
        }
         
    }