服务器在执行多个主流php程序时报错，警告如下：

Warning: Unknown(): open(D:\php\sessiondata\sess_da5315b3b42626359f461bbbf91bb299, O_RDWR) failed: Permission denied (13) in Unknown on line 0

Warning: Unknown(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (D:\php\sessiondata) in Unknown on line 0

请问是什么原因？怎么解决？