In a dev environnement I tried to update from 3.0.12 to 3.1-RC4 using this step :

1) Downlaod the phpBB-3.0.12_to_3.1.0-RC4.zip
2) Unzip it on a local folder
3) Upload the 3 folders and all files to the phpBB folder using FTP
4) Launch the install.php file from the install folder's of the phpBB
5) Following step and successfully upgrading files
6) Clicking the "Update my database now" button which call http://SITE_DOMAIN/PATH_TO_PHPBB/install/database_update.php?type=1&language=fr (also tried with the english language pack) giving a blank page. Going to the server access log I see there's a 500 error (see image in attachment).

The error is : Parse error: syntax error, unexpected end of file in functions.php on line 5469