title: Check if property has been set in phing
body: <p>How to check if property <code>${foo}</code> has been set?</p>
  
  <p>Bonus question: how to <em>escape</em> <code>$</code> sign in <code>&lt;echo&gt;</code> so I could output <code>${foo}</code> string (not the <code>foo</code> variable substitution)?</p>
  
  <p>PS: tried to google and read documentation, but couldn't find the answers. It's likely I'm missing something</p>