Site admin doesn't work after update

I just updated a site to 6.16. After doing so, we started getting a bunch of errors that seem to point to us not having the right version of PHP. However, our hosting is on 5.2.9.

First we got errors on imageapi, so I turned that off.

Then we got this error:

Fatal error: Call to undefined function: array_intersect_key() in /home/username/public_html/modules/update/update.compare.inc on line 695

Everything on this I could find mentioned using PHP4, but our Cpanel says we are using 5.2.9.