title: What does using a dollar sign after $this-> in PHP mean?
body: <p>I'm a little confused by some PHP syntax I've come across.  Here is an example:</p>
  
  <pre><code>$k = $this-&gt;_tbl_key;
  
  if( $this-&gt;$k)
  {
     $ret = $this-&gt;_db-&gt;updateObject( $this-&gt;_tbl, $this, $this-&gt;_tbl_key, $updateNulls );
  }
  else
  {
      $ret = $this-&gt;_db-&gt;insertObject( $this-&gt;_tbl, $this, $this-&gt;_tbl_key );
  }
  </code></pre>
  
  <p>My question is basically what does <code>$this-&gt;$k</code> mean?  I figured it might mean the member variable that goes by the name of whatever is in <code>$this-&gt;_tbl_key</code>, but how would that work?  Is it possible to add member variables to a class at run-time?</p>