check unique - laravel

When a user registers, each user receives a seven-digit code stored in the database. Each code must be unique and must not be created a second time. How can I ensure that each code stored is unique and that no error messages occur when an existing code is created?

Function

 $randomstring = substr(str_shuffle("0123456789abcdefghijklmnopqrstuvwxyz"), 0, 7);

Table

Schema::create('invites', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned()->nullable();
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->text('greeting')->nullable();
            $table->string('url')->unique();
            $table->timestamps();
        });