PDFlib 6 and PHP 4.3.8/9

Hi Everyone, I am hoping someone out there may be able to help... I have recently installed PDFlib 6 and am running PHP 4.3.8. I am having trouble with the pdf_open_file() function. I would like to create a PDF to memory by leaving the second parameter (filename) in the above function empty. However, every time I do this, I get the following error message: Warning: pdf_open_file() expects exactly 2 parameters, 1 given in xxxxxxx This means I always have to create the PDF file before outputting it to the browser... which is very annoying. Has anyone else encountered this? I have been unable to find much in the groups to this point! Thanks in advance for the help. -brendan