title: PHP parser: braces around variables
body: <p>I was wondering, how is the semantics of braces exactly defined
  inside PHP? For instance, suppose we have defined:</p>
  
  <pre><code>$a = "foo";
  </code></pre>
  
  <p>then what are the differences among:</p>
  
  <pre><code>echo "${a}";
  
  echo "{$a}";
  </code></pre>
  
  <p>that is, are there any circumstances where the placement of the
  dollar sign outside the braces as opposed to within braces makes
  a difference or is the result always the same (with braces used
  to group just about anything)?</p>