Panel: Destroy fields after saving

I’ve set up a box running CentOS 7 + LAMP (PHP5 Latest) and installed kirby.
Everything is working, except when I try to edit a page via panel, the following occurs:

1 - Login in the panel
2 - Edit any page fields
3 - Click save
4 - Every field becomes 'blank’
5 - All field in the equivalent .txt file transforms from

This: Hello 
----
Other: Ok

to

T: Hello
----
O: Ok

What am I missing? It took me a long time to detect this, I thought it was a problem with permissions.
I have no clue why this is happening.

The mbstring extension seems to be missing …
http://getkirby.com/docs/installation/troubleshooting

The resource was installed. It came along with libapache2. I checked it by running php -m

It just needed to be activated. Now it is working, thank you.