I have opened a very small PR at https://github.com/php/php-src/pull/11487
I was told to send an email here and ask for feedback.
This PR adds support for rounding negative |$decimals| in |number_format()|.
Previously negative |$decimals| got silently ignored and the number got
rounded to zero decimal places.
With this change, when |$decimals| is negative, |$num| is rounded to
|$decimals| significant digits before the decimal point.
This is matching the behavior for the |$precision| argument of |round()|.
The previous behavior wasn't documented or tested at all.
if there are no objections, Ilija has agreed to merge this for master.
We will of course document this accordingly.