src/includes/traits/CacheMembers.php
protected function cacheAddByRef(string $primary_key, $ref_key, &$value)
    {
        $ref_key                                               = (string) $ref_key;
        $this->x___cache['x___refs'][$primary_key][$ref_key][] = &$value;
    }
protected function cacheAddByRef(string $primary_key, $ref_key, &$value)
    {
        $ref_key                                               = (string) $ref_key;
        $this->x___cache['x___refs'][$primary_key][$ref_key][] = &$value;
    }
Add a new cache ref-key.

@since 150424 Initial release.

@param string     $primary_key Primary key.
@param string|int $ref_key     Ref-key to set.
@param mixed      $value       New value (by reference).