For the life of me, I am unable to perform an upsert and get a lookup field to get populated.  I am getting the error "id value of incorrect type: 3".  Here's how I have things configured.

Custom object 1
External_Id__c - this is a number field and has the attributes External ID and Unique checked to true. (example value is 3)

Custom object 2
Has a lookup field to Custom Object 1.  When upserting, my PHP array sets this values to a value that exists in CustomObject1.External_Id__c (example value is 3)
Upserting a record here gets the error "id value of incorrect type: 3"

If I comment out the lookup field when upserting to Custom Object 2, a record is upserted with no errors.

Any ideas? Do I have to do anything special in the PHP code to get this foreign key relationship to work?