lace as atomically as possible known value to new one
	 */
	public function set_if_maybe_equals( $key, $old_value, $new_value ) {
		// cant guarantee atomic action here, filelocks fail often
		$value = $this->get( $key );
		if ( isset( $old_value['content'] ) &&
			$value['content'] != $old_value['content'] )
			return false;

		return $this->set( $key, $new_value );
	}

	/**
	 * Use key as a counter and add integet value to it
	 */
	public function counter_add( $key, $value ) {
		if ( $value == 0 )
			return true;

		$fp = $this->fopen_write( $key, '', 'a' );
		if ( !$fp )
			return false;

		// use "x" to store increment, since it's most often case
		// and it will save 50% of size if only increments are used
		if ( $value == 1 )
			@fputs( $fp, 'x' );
		else
			@fputs( $fp, ' ' . (int)$value );

		@fclose( $fp );
		return true;
	}

	/**
	 * Use key as a counter and add integet value to it
	 */
	public function counter_set( $key, $value ) {
		$fp = $this->fopen_write( $key, '', 'wb' );
		if ( !$fp )
			return false;

		$expire = W3TC_CACHE_FILE_EXPIRE_MAX;
		$expires_at = time() + $expire;

		@fputs( $fp, pack( 'L', $expires_at ) );
		@fputs( $fp, '