title: Usage of "isset" function
body: <p>Although I visit php documentation , I didn't understand usage of "isset" function .
  1 - for example in a php tutorial book author wrote a text with this context : when we create a form in a first.html and we want to use from form information in second.php , we must use these 2 pieces of code :</p>
  
  <pre><code>if(!isset($_GET['q']))  
         die("The Search key word is not set !!!"); 
  $key = $_GET['q'];  
  if($key == "")  
         die("The Search key word must be entered !!!"); 
  </code></pre>
  
  <p>but I don't understand what is difference between these 2 codes ?</p>
  
  <p>2 - for another example I saw this code for checking that a bottom is clicked or not :</p>
  
  <pre><code>if(isset($_POST['login_btn']))  
  {  
     ....
  }
  </code></pre>
  
  <p>but I don't understand why does it check that a bottom is clicked or not ?</p>