$('#soff_gar').select2({placeholder: "Scegli", initSelection: true }).select2("val","<?php echo $garante; ?>");

$('#soff_gar').select2({placeholder: "Scegli", initSelection: true }).val("<?php echo $garante; ?>");

$('#soff_gar').select2({placeholder: "Scegli"}).val("<?php echo $garante; ?>");

<select id="mySelect">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

$("#mySelect").select2();
$("#mySelect").val("audi").trigger("change");

$('#soff_gar').select2({
  placeholder: "Scegli",
  initSelection: true
});
$('#soff_gar').val(<?php echo $garante; ?>).trigger("change");

<option value="audi" selected>Audi</option>

var regex = /([^=&?]+)=([^&#]*)/g, params = {}, parts, key, value;

while((parts = regex.exec(url)) != null) {
    key = parts[1], value = parts[2];
    var isArray = /[]$/.test(key);

if(isArray) {
        params[key] = params[key] || [];
        params[key].push(value);
    }
    else {
        params[key] = value;
    }
}

return params;  }