It's a Ubuntu Package BUG!
yesterday night I update php5-common and libapache2-mod-php5 to 5.2.4-2ubuntu5.13....
all php were in blank ...
this is the bug: https://bugs.launchpad.net/ubuntu/+source/php5/+bug/701765
I manually install 5.2.4-2ubuntu5.12 to fix the server.
this morning...