title: Isset vs array_key_exists
body: <p>Where is <code>isset</code> or <code>array_key_exist</code> suitable to use ?</p>
  
  <p>In my case both are working.</p>
  
  <pre><code>if( isset( $array['index'] ) {
     //Do something
  }    
  
  
  if( array_key_exists( 'index', $array ) {
     //Do something
  }
  </code></pre>