I don't know much about PHP I'm afraid and it's hard to know where to look without more details of your problem. However, 2 thoughts:
1) Have you tried this simple script in order to get a lot of information about PHP and how it is configured on your system? Create a file with (just) the following:
<?php phpinfo() ?>
You can then execute this from your web browser or from the command line.
2) Problems with CRON jobs are most often related to the user environment in which they are running - mostly environment variables that are setting the user's path, default langauge, timezone etc. Try printing out the values from the script in CRON to check they are what you expect them to be.