file_put_contents won't write data anywhere

I have some php scripts that dump certain data to disk so that a user can later, optionally, download them. This worked fine on my local development machine but is not working on my DigitalOcean droplet.

Boiled down, the code looks like

<?php
 file_put_contents("./onedot.txt" , "?");
 file_put_contents("../twodots.txt" , "?");
 file_put_contents("./pro/dotslashpro.txt" , "?");
 file_put_contents("/slash.txt" , "?");
 echo 'tried it';
?>

The code runs but none of these files get created

an alternate implementation based around fopen() and write() resulted in a “can’t open file” error.

I presume there is some setting in Apache or in Debian that I need to change. Any suggestions?