title: How to create foreign key by Laravel migration?
body: <p>How can we make a <code>referenced key</code> and <code>foreign key</code> in <code>laravel</code> by <code>migrations</code>.
  Think I have two migration files in database directory in laravel, both of them create a different table in my database.
  the first migration creates a table that is for posts which has a column in the name of Id.
  the second migration create comments table that has a column in the name of post_id. Now <code>the Id column in posts table is referenced key</code> and the <code>post_id in comments table is foreign key</code>, how can I connect these two columns together?</p>