Check you cron job. Make sure the path are okay.
This is likely due to Plesk using a chrooted environment. The user that is executing the script doesn’t have access to PHP. You have a few options:
1. Add PHP to the user’s chroot – http://kb.sp.parallels.com/en/115842
2. Disable the chrooted environment entirely – http://blog.blums.eu/2013/01/25/parallels-plesk-panel-11-non-chrooted-cronjobs
3. Run the script as root because root does have access to PHP (not recommended but if the script is trusted it might be ok).