what is the meaning of ${$key} in the code below?
<p>I just need to know what is the meaning of <code>${$key}</code> in the code. I already searched google but didn't find answers for this code. So please help me understand it ?</p>

<pre><code>&lt;?php
foreach ($_POST as $key =&gt; $value) {
    $temp = is_array($value) ? $value : trim($value);

    if (empty($temp) &amp;&amp; in_array($key, $required)) {
        $missing[] = $key;
        ${$key} = '';
    } elseif (in_array($key, $expected)) {
        ${$key} = $temp;
    }
}
?&gt;
</code></pre>