Payment Processing - Credit Cards - Any Libraries available?

Regarding PCI compliance  -
If I build site with codeignitier and use authorize.net, and then the site will be hosted with a hosting company.

In that case, do I only need to make sure the hosting company's web server is PCI compliant?

I'm sorry Tom but I honestly do not recall all the requirements/steps to be PCI compliant. I do remember is was a can of worms. As my brush with PCI was 6+ years ago I imagine that the process has changed anyway.