Very Strange Problem: No PHP Sessions and /tmp is Empty!

[mss6]

I'm facing a strange error with my server. All the sudden, PHP scripts cannot create trace transactions. For example, login info is not kept between pages, etc. The scripts don't even give any error messages.

I checked /tmp directory and found empty. There is only one link (mysql.sock -> ../var/lib/mysql/mysql.sock=) in it.

I rebuilt Apache but that did not solve the issue.

Any help please?

Thank you.

[webignition]

Since this issue is also being discussed in http://forums.cpanel.net/showthread.php?t=46273, it might be best to continue there so that any useful information can be grouped together for others facing this issue.

[mss6]

I updated that post.

Mysql is also affected. Here is an example:

Invalid SQL:
SELECT setting.*, settinggroup.grouptitle
FROM settinggroup AS settinggroup
LEFT JOIN setting AS setting USING(grouptitle)
WHERE settinggroup.displayorder <> 0
ORDER BY settinggroup.displayorder, setting.displayorder

mysql error: Can't create/write to file '/tmp/#sql_4aca_0.MYI' (Errcode: 13)

mysql error number: 1