Website A = Website B
Script to import CSV file on Website A = works
Script to import CSV file on Website B = !works
Clock = 4am
Me = null

There's something wrong with you not the script

Maybe no write privileges xd

You need more coffee

Turned out to be an obscure admin user setting in the database. Really simple fix that took hours to discover. :(

ENV A != ENV B

Guess the first equality does not hold, since equality = substitutability 😢
May the force always be with you, without exceptions.

Are u parsing CSV using frontend JS, if so, r there any good/small libraries/code for that?

https://npmjs.com/package/json2csv/
This might help

Thanks for that tip, @armeg. It's more basic than that. An export of a DonorPerfect "database" (essentially a glorified spreadsheet) to CSV and then a custom PHP script to conform the data to certain rules and import it to WordPress usermeta and user tables as part of a paywall membership system. Except after all this work it turned out not to be anything close to the new requirements they suddenly insisted they were talking about "the whole time".