Find array key where value contains a specific value
<p>I have an associative array where each value is a list of numbers, like this:</p>

<pre><code>$a['i']=[111, 333];
</code></pre>

<p>Given the value <code>333</code>, how can I find the key <code>i</code>. That is, the key for a list which contains <code>333</code>.</p>