Why does the "session.save_path" has no value on SunOS 5.1?

I use the same php.ini in 4 platforms (Linux, AIX, HP UX and 开发者_JS百科SunOS), which the session.save_path has been configured with value "/tmp". Doing a phpinfo(), all platforms show the correct value (/tmp) except for SunOS 5.1, which is showing no value. Any idea what might be the reason?

Maybe the version of PHP that is installed on SunOS 5.1 does not have that option available, or it is called something else on that version?