If you call a bash script from a php one, be sure that you have the correct permissions. To grant them, you could have to do this command:
setfacl -m u:www-data:rwx /the/path/where/you/have/to/exec/bashlinux: keep open the terminal
you can add at the end of your command ;bash
for example:
rclone sync mega:ebooks /mnt/zippati/ebooks/ebooks/ -l -v ;bash