Move from MySQL to PostgreSQL

Hello, I try to move from MySQL to PostgreSQL.

All seems to work expect of one query.
The query is part of https://github.com/volkszaehler/volkszaehler.org

https://github.com/volkszaehler/volkszaehler.org/blob/master/lib/Util/Aggregation.php

There are several problems.

One of that ist the 'REPLACE INTO'.
An other problem is '@prev_timestamp := timestamp'

I have no idea how this can rewritten, so that MySQL and PostgreSQL can
understood that.
For DATE_FORMAT and UNIX_TIMESTAMP I have found wrapper functions.

Can someone please help me?

Best Regards,
Basti