If you pass the post request with following parameter to your Laravel controller you will not get the value for today
from : ‘2019-01-01′
to:’2019-01-01’
PurchaseOrder::whereBetween('created_at', [$request->from, $request->to])->sum('total')
So you need to change the code following way
public function getPurchaseOrderTotal(Request $request){
return PurchaseOrder::whereBetween(DB::raw('date(created_at)'), [$request->from, $request->to])->sum('total');
}