PHP parser: braces around variables
<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>
