please explain following out put : <p>Here, I have following code it getting output like 21,21. How 21 value comes in $a</p>

<pre><code> $a = '1';
 $b = &amp;$a;
 $b = "2$b";
 echo $a.", ".$b;
</code></pre>

<p>Output</p>

<pre><code> 21,21
</code></pre>
