public function addIdentifier($identifier)
    {
        if (!isset($this->identifiers[$identifier])) {
            $this->identifiers[$identifier] = $identifier;
        }

        return $this;
    }
public function addIdentifier($identifier)
    {
        if (!isset($this->identifiers[$identifier])) {
            $this->identifiers[$identifier] = $identifier;
        }

        return $this;
    }
public function addIdentifier ( $ identifier ) { if ( ! isset ( $ this -> identifiers [ $ identifier ] ) ) { $ this -> identifiers [ $ identifier ] = $ identifier ; } return $ this ; }
Adding a identifier to the list.

@param string $identifier contact identifier

@return self
Adding a identifier to the list .