One thing to keep in mind is that when you run PHP from cron, you're running the binary instance of PHP, not the Apache module.
You can run the shell command "php -v" (or "/usr/bin/php -v", etc.) to check the version for the one that cron uses.
Another factor is that when running PHP from cron, rather than running it from the shell, the environmental parameters, such as the current working directory, may be different.
When I was setting up my PHP cron stuff, I first tried it with a simple test script that merely printed out the PHP version, current working directory, etc.
