xslt_create occasionally generates Fatal Error: Unknown function

[csbdeady]

Hi
Although my install of PHP has Expat enabled, when I execute:

	$parser = xslt_create();

The "first" time (defined as first time I load my web browser and point to the applicable PHP page on the internet, or after approx. 30 mins of inactivity if I reload the page) I do this I see:

Fatal error: Unknown function: xslt_create() in <filename> on line <n>

However if I immediately refresh the page no error is generated and I see gloriously formatted HTML (XML -> XSLT -> HTML)

What could be causing xslt_create() to fail? I do have Expat installed according to phpinfo():

XML Support active
XML Namespace Support active
EXPAT Version expat_1.95.7

I have asked someone else to try a different web browser on their computer, and they also see the same error once, and a perfectly formatted page upon reloading it.

Thanks
-Colin