default value php

I'm trying to write some default value php for a select list text field with three allowed values. The first snippet of code I tried did not successfully change the default value - in node edit pages, the first allowed value appeared, no matter what value I selected for the default. I wrote the simplest bit of code possible

<?php return array(0 => array('old' => 'Old')); ?>

and was still unsuccessful. The first allowed value keeps showing up, no matter what.

Apologies if the answer is obvious or this is a duplicate issue.