Cronjob没有在Ubuntu 16.04上工作

I would like to put in place some cronjobs for my website.

My actual cronjob is: 4 * * * * /usr/bin/php /var/www/domain/backup.php

But it doesn't work.

• Mannually the cron job is working.
• With a service like https://www.easycron.com it's working too.

In Terminal, when I make crontab -e, I see my cronjob but it does not seem to run.

From where I can start my search to understand ?

Thanks.

In my server, when setting up a php file to run, I only have to put: 4 * * * * php /var/www/domain/backup.php. They run flawlessly. Works the same for my shell scripts as well with just bash /directory/thisScript.sh

I added this as an answer because my reputation score won't let me add comments yet.