title: Error when comparing the values of the same variable
body: <p>I'm having a little problem with validation. I have here this code snippet that works by this way: if the <code>pizza_size</code> variable is not set in the Query String and the ID of product category is 1 (corresponds to pizzas) or this is set but the <code>pizza_size</code> value is not equal to <code>small</code>, <code>medium</code> or <code>big</code> this needs to return to the page <code>products.php</code> with the message that the size is not selected. I'm trying with:</p>
  
  <pre><code>if(!isset($_GET['pizza_size']) || $_GET['pizza_size'] != "small" || $_GET['pizza_size'] != "medium" || $_GET['pizza_size'] != "big" &amp;&amp; $category_id == 1)
  {
      redirect_to('products.php?message=no-size-informed');
  }
  else //Add the product
  </code></pre>
  
  <p>The problem is that this is not working. Even if the <code>pizza_size</code> corresponds to <code>small</code>, <code>medium</code> or <code>big</code> I receive the error message. But if I use only one comparison, this works great, like:</p>
  
  <pre><code>if(!isset($_GET['pizza_size']) || $_GET['pizza_size'] != "small" &amp;&amp; $category_id == 1)
  {
      redirect_to('products.php?message=no-size-informed');
  }
  </code></pre>
  
  <p>But I need to validate with the 3 sizes. How to do this validation with all the three possible variables? Thanks!</p>