Conversion between ISBN-10 and ISBN-13

Hi,

Can anyone tell me what the conversion between ISBN-10 and
ISBN-13 is,
please.  I need to write a little conversion program. 
Anything in PHP,
for example.

Thanks a lot.

David Kane
Waterford Institute of Technology
http://library.wit.ie/
T: ++353.51302838
M: ++353.876693212
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