I have a temporary measure. Hopefully it helps in some way. I was having a similar issue with a Tiny Tiny RSS deployment. The actual application worked fine, I could navigate around my feeds etc., but the feeds wouldn't update due to the same PHP errors.
My feeds update via a cron job that would just call the .php file directly. So in this case my cron was performing the following:
$OPENSHIFT_REPO_DIR/php/update.php --feeds
The above used /usr/bin/php as you had stated above by default. I changed the above to:
/opt/rh/php54/root/usr/bin/php $OPENSHIFT_REPO_DIR/php/update.php --feeds
Which fixed the issue. Hope this helps in some way...