$name = $hook->getValue('name');<select name="source[]" multiple="multiple">
<option value="Google" [[!+fi.source:FormItIsSelected=`Google`]]>Google</option>
<option value="Facebook" [[!+fi.source:FormItIsSelected=`Facebook`]]>Facebook</option>
<option value="Twitter" [[!+fi.source:FormItIsSelected=`Twitter`]]>Twitter</option>
</select>
$source = $hook->getValue('source');
Hi, I've never done a posthook, but the name of your field is source[] and not source. Have you tried to change this ?
$source = $hook->getValue('source'); doesn't work
$source = $hook->getValue('source[]'); doesn't work
$source[] = $hook->getValue('source[]'); doesn't work
$source[] = $hook->getValue('source'); doesn't work