What is the different between {$hello}, ${hello} & $hello when use double quotes?
<p>I'm a bit confuse with </p>

<pre><code>$hello = "hello";
echo "Say $hello";
echo "Say {$hello}";
echo "Say ${hello}";
</code></pre>

<p>and the output is same <code>Say hello</code>. When should I use <code>{$hello}</code> and <code>${hello}</code>? and why it cannot be used in single quote?</p>
