Laravel dynamic where queries using Query Builder

I am in new in Laravel. I want to make dynamic where queries with laravel query builder.
Normally I can make dynamic queries in php

$where = array(
  'hello' => 'world'
);

function get($where = null){
   if($where == "")  $where = "";
  //Function which converts where clause into queries
  wheretoqueries($where);  //converts where clause
  $sql = "SELECT * FROM $tbl $where";
  return $sql; 
}
echo get($where);

If where clause is null queries will be

SELECT * FROM $tbl

If where clause is not null queries will be

SELECT * FROM $tbl WHERE hello = "world"

Laravel orm works fine for where clause if key and value exists

A::where($where)->get();

If where is null following method will not work

via Chebli Mohamed