how to concatenate custom fields on form submit

I’ve developed a form for submitting content without being logged into the site. The form and post submission are working but, in order for the plugin that creates the map on the post to work, I need to combine 2 custom fields (address and zip) into one field.

I have this in the form:

<td><fieldset><input type=”hidden” id=”_ct_text_4e40d58483b14″ name=”_ct_text_4e40d58483b14″ value=”<?php echo $_POST[‘_ct_text_4e3319808b191’] . $_POST[‘_ct_text_4e33216358869’];?>”></fieldset></td>

The values of the individual custom fields are populated but the concatenation is not working. If anyone has an idea how I could/should be writing this code, it would be most helpful.