1. create an application of Tiny Tiny RSS, named tt-rss
2. git clone ssh://******************************************
3. cd tt-rss
4. delete everything in php folder and unzip selfoss-2.10.zip in php folder instead.
5. configure the default.ini by the given pgsql info from openshift, e.g.
db_type=pgsql
db_host=127.***.***.***
db_database=ttrss
db_username=adminn******
db_password=************
db_port=5432
6. git push and etc.
7. access https://ttrss-******.rhcloud.com/, done~

(8.) add these to .htaccess in order to force HTTPS
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
</IfModule>

However, I cannot fix a serious problem: it shows the connection is only partially encrypted. This problem does not occur on Tiny Tiny RSS.

Thus I give it up and come back to the selfoss on my VPS at the end.