How do you make an etag that matches Apache?

Question

I want to make an etag that matches what Apache produces. How does apache create it\'s etags?

Answer 1

the answer above (from Chris) works well, but can be simplified using an implicit cast in the sprintf:

sprintf('"%x-%x-%x"', $s['ino'], $s['size'], str_pad($s['mtime'], 16, "0"));

The suggested %016x doesn't work because the padding is applied after the conversion to hex, rather than before.