Adsense block yields broken site

Hi,
I am trying to install an adsense module on a drupal 5.20 site and when I tell the block to appear in the left column, the module throws an error:

Fatal error: Call to undefined function: stripos() in /homepages...htdocs/modules/adsense/adsense.module on line 542

I can fix this by using the webhosts interface to move the module out of the modules dir, and into another, but I can't begin to deal with the problem. I have been searching and reading about it, and would guess that it is needing php 5, and can't get it, but my hosts package says it has php 5.

Can anyone throw me any ideas please? The host is 1and1 and the site, as ever, is alternicity.com

cheers in advance,
Stuart