posix_getpwnam

Hi,
 I`m using function posix_getpwnam. Its works correctly. But The result for
param passwd is only "x". I know why is it. Because i have save unix users
passwords in shadow file,not in passwd file, there is only "x".
 Is there some function,which param is encrypted(MD5) password in PHP?

I need: User puts password,and I will compare these password with password
in shadow?

Thank you very much

Tomas