Both fields are equal to null value, can't display error message
<p>I'm developing a booking system, customers can book a maximum of 3 seats. If they book only 1 seat, this means that the 2 other seat values are set to null, when I submit the form I check it to make sure a customer isn't trying to double book the same seat, as you will see in the code below, so if the customer buys 1 seat, they'll get an error because both seats are equal to null, how can I solve this? Thanks </p>

<pre><code>if($seat1==$seat2 || $seat2==$seat3 || $seat1==$seat3){
echo("You're trying to book the same seat numerous times, please go back.");
die();
}
</code></pre>
