Make sure you are asking for the custom field by it's field name (it will be "f" followed by digits).
To find all the field names in an account, visit https://api.ontraport.com/live/ and enter an APP ID and API Key from the account (you can generate these in Administration > Integrations > ONTRAPORT API Key Manager).
After the APP ID and API key load - the boxes will turn green indicating a live connection - click on the first link, "Objects" and then on GET /objects/meta. When the section expands click the "Try it out!" button. The response body will contain all of the fields in the Contacts.
The field names and aliases are shown in the response body:
Our PHP SDK is located at Github (click on the "Find File" button to see the code examples).
If you still need help, please include the code you are using in a text file, along with the any error message you receive, and attach it to an email sent to firstname.lastname@example.org. In the email, please provide the account number and API Key and APP ID you are using.