Check the permission of the directory, make sure the web server process is able to write files in that dir. The Apache is usually running as user apache and group apache. If the .../session/tmp looks like:

drwxr-xr-x 23 armint users   1024 Jun 15 13:42 tmp
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then you have a problem. The permission scheme (drwxr-xr-x or 775) and user/group ownership (armint/users) disallows user/group apache to write on that dir. If you have privileges to change the ownership, then change it so the owner of that directory is apache (but this doesn't prevent your session files from reading by other means). If you don't, and you don't have (or unable to get) someone with required privileges to do so, the alternative is by changing the permission to 777 (gasp!!! highly discouraged).

Or, just use the common/shared temporary directory (usually /tmp on Linux system), but this means your session files are readable by other users on the system (the same applies with 777-permission).