PHP quotes in SQL : <p>Why does this code produce an error</p>

<pre><code>  $sql = "INSERT INTO accountlist VALUES ("", "$user", "'$pwd", "$mail", "$date")";
</code></pre>

<p>and this one doesn't?</p>

<pre><code>  $sql = "INSERT INTO accountlist VALUES ('', '$user', '$pwd', '$mail', '$date')";
</code></pre>

<p>I know that double quotes process variables while single quotes doesn't, so the first option should be the right one, but it is the opposite!</p>
