set CCK field permissions per PHP in Drupal 6

I want to change the field level permissions from a CCK field by PHP in my custom module. Can You give me a hin开发者_如何学Got?

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Implement hook_field_access().

See http://drupalcode.org/project/cck.git/blob/refs/heads/6.x-3.x:/modules/content_permissions/content_permissions.module for an example.