Now, we don't have all that much to go on but, the first thing you should do is look in your server error log. If you are using apache, then the log is normally:

/usr/local/apache/logs/error_log
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Of course, your configuration may be different, but find out where it is logging errors. Then, run your script, and straight after the error occurs, look at the last few entries in the log. If your server is a unix machine, then the command:

tail /usr/local/apache/logs/error_log
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will do the trick. Now sometimes these messages are not blindingly obvious, but this will at least point you in the right direction.