code: public function up()
      {
          Schema::table('charges', function (Blueprint $table) {
              // Linking
              $table->integer('plan_id')->unsigned()->nullable();
              $table->foreign('plan_id')->references('id')->on('plans');
          });
  
          Schema::table('shops', function (Blueprint $table) {
              // Linking
              $table->integer('plan_id')->unsigned()->nullable();
              $table->foreign('plan_id')->references('id')->on('plans');
          });
      }
docs: Run the migrations.
  
  @return void