I decided to just set up the same cron job using your script. Hope you do not mind.
I get this back from crond, complete with your typo
Database back-up completd successfully
So it certainly seems to run okay as you have laid it out.
The filename is as you expect
I have just let the crontab run and I have had another OK and the gzipped file produced.
Sorry I cannot think of anything further to add. You are running the Bash shell? You have chmodded script.php to 755? You have added the extra linefeed to the crontab?
I have not tried under PHP5 but I would not see it being different.
Maybe one for support just in case there is a problem with tmp file permissions.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!