php - write failed: No space left on device (28)

Your / is full and you are attempting to save to /opt/... which is on / too. You'll have to find what is filling your / and free up some disk space.

You can find what's using your disk space with something like this

cd /
find . -maxdepth 1 -exec du -sh {} \;

which will list the size of each top level directory. You can then drill down using the same find command.

/dev/mapper/VolGroup-lv_root 50G 47G 0 100% /

... session.save_path is correct (/opt/lampp/tmp/session) ...

Ack. Your session save path is on /, which is full.

As an alternative to @Iain's command, you can use du -m |awk '$1>5000' to only list directories occupying more than 5GB, but not limited to the top level. Depending on how deep in the tree the culprit is, this might save you some typing (of cd xx && du ...)