You should change the PHP code with following code

<div class="col-md-12">
            <div class="form-group first">
                <label >Date_reservation</label>
                <input type="date" name="date_res" id="date_res">
            </div>
        </div>
        <div class="row">
            <div>
            <div class="form-group first">
                <label >Type_salle</label>
                <select name="code_salle" id="code_salle">
                   <option>STD1</option>
                   <option>STD2</option>
                   <option>STD3</option>
                   <option>STD4</option>
               </select>
           </div>
        </div>
        </div>

After that,You can use onchange function to fetch room details from the database(fetch_room_data.php, which is used to fetch the room table details from the database). Here, I will provide a sample code

$('#date_res').on('change', function() 
{                                                                  
$.ajax({
url:"fetch_room_data.php",
method:"POST",
success:function(data)
{ 
  $('#code_salle').html(data);
}
});
});

You will need Javascript and an API Route, because you can't just connect html with your database that's not how it works.

An alternative option would be to use blade(a template engine for laravel).