laravel migration add constraint without creating new migration code example

Example 1: laravel migration foreign key 5.6

Schema::table('posts', function (Blueprint $table) {
    $table->unsignedInteger('user_id');

    $table->foreign('user_id')->references('id')->on('users');
});

Example 2: laravel drop table migration

Schema::drop('users');

Schema::dropIfExists('users');