error_log() problem

I have been trying to write a custom error handler (or at least adapt the one used in O'Reilly's PHP and MySQL book) and I'm encountering an issue with the error_log() php function. I'm not sure whether this is a problem in my configuration files or whether the function is being used incorrectly - I haven't altered this part of the code from the book. The code being called is:

error_log($error, 3, ERROR_FILE);

And it produces the following error message (file paths removed):

Warning: error_log(...filepath.../log/php_error_log.log): failed to open stream: Invalid argument in ...filepath.../phpincludes/customHandler.inc on line 89

If anyone can help me (I've spent hours trawling Google with no luck) it would be much appreciated.

Life is tough when you can't debug your debugger...