1.5.x Upgrading from 1.5.0 to 1.5.5a with question about which PHP version to use

In a ZC support doc, it says that "v1.5.0 is compatible with PHP 5.2.14 thru PHP 5.3.x." I am running PHP 5.4, and the site hasn't experienced any issues whatsoever. However, I only have one server to run both the live and dev sites. I can upgrade the server to PHP 5.5, so I am wondering if I should do my upgrade work running PHP 5.4 or 5.5. Meaning the live site would go to 5.5, too, if I tested 1.5.5a with PHP 5.5 running. Or I would be testing 1.5.5a on PHP 5.4 while the live site runs as it always has. Any thoughts or recommendations? Thanks.

ZC 1.5.5 (including a) is compatible with PHP 5.4. Further though it is ideal to maintain the most recent software, use of PHP 5.5 is likely to disable your live store (assuming that it is not overly modified to support higher versions) and that would be counter-productive...

Upgrade the ZC software as is, then once functional upgrade the PHP version to the highest available.

mc12345678:

Upgrade the ZC software as is, then once functional upgrade the PHP version to the highest available.
Right. Since v155 is compatible with your old PHP 5.4, do your complete upgrade while still running PHP 5.4. Then after the upgrade you can switch to a newer PHP version.

so I am wondering if I should do my upgrade work running PHP 5.4 or 5.5
do your complete upgrade while still running PHP 5.4
Yes, one problem at a time.
Once you have done your upgrade and your production shop is stable, I would recommend you set up a local development server using php7 and spend your time getting your shop working under that rather than 5.5.
ZC155 is fine with php7 but your mods wont be, but the debug errors are straightforward to fix and then your shop will be fine for the next few years.
This is what I have been doing recently, so I wont have to worry about my host doing php upgrades for a couple of years.

Okay, thanks, everyone. I was hoping for the answers given.