Enable field when selecting from drop-down box
<p>My contact form page, for user edit details, has two drop-down fields, ‘country’ and ‘city’.
I would like when a user is edit his details that the ‘city’ field will be disabled until something is selected in the ‘country’ drop-down menu.</p>

<pre><code>&lt;form name="item" action="&lt;?php echo base_url(true) ?&gt;" method="post"&gt;

&lt;label&gt;&lt;?php _e('Country', 'my_theme'); ?&gt;&lt;/label&gt;
&lt;?php ItemForm::country_select(get_countries(),user()) ; ?&gt;

&lt;label&gt;&lt;?php _e('City', 'my_theme'); ?&gt;&lt;/label&gt;
&lt;?php ItemForm::cities_select(get_cities(),user()) ; ?&gt;

&lt;button class="itemFormButton" type="submit"&gt;&lt;/button&gt;
&lt;/form&gt;
</code></pre>

<p>I’ve tried ‘<code>onchange</code>’ in javascript, probably with wrong syntax…</p>

<p>How can I create this? 
Thx.</p>
