How do I reference another field properly? In Drupal 7, how can I properly reference another field with the Computed field module?
It's best to turn on the devel module and do a dsm() on the $entity object to see what fields are available to you. Even if you think field_name would work, there are other arrays inside that field variable. 

In the case below i am parsing a URL link from another field and exploding it via the dashes. Since my field_sell_link is a "link field" i had to add the ['url'] at the end of the field reference instead of the usual ['value']. I didn't find this out until I did a dsm on $entity. You can also do a set_message(print_r($entity)); too.

let me know if this helped anyone.

Example code for the PHP code section:

$ebay_url = isset($entity-&gt;field_sell_link['und'][0]['url']) ? 
  $entity-&gt;field_sell_link['und'][0]['url'] : 
  'zero/one/two/three/nothing-here-asdf/last';

if (isset($ebay_url)) {
  $url = explode('/', $ebay_url); 
  $pattern = '/-/'; 
  $replace = ' '; 
  $entity_field[0]['value'] = 'testxxx' . $url[3];
} 
else {
  $entity_field[0]['value'] = "";
}