system/classes/Collection.php
public function fetchOffsetKey($num)
    {
        // Keys for resources are always strings. If given a numerical offset, 
        // that offset needs to be interpreted to determine the actual property's name
        // at that offset.
        return isset($this->contentKeys[$num]) ? $this->contentKeys[$num] : null;
    }
public function fetchOffsetKey($num)
    {
        // Keys for resources are always strings. If given a numerical offset, 
        // that offset needs to be interpreted to determine the actual property's name
        // at that offset.
        return isset($this->contentKeys[$num]) ? $this->contentKeys[$num] : null;
    }
Returns the offset name (string) of a resource for a given numerical offset.

@param num A numerical offset.