What is in your /home/<username>/public_html/<domainname>/php.ini file? Anything unusual? From the messages, it sounds like something in the WP install process is running, so I don't think you're locked out of PHP entirely.
What is the history of this site? Was there another application there before, or is it brand new? Did you manually create the directories (cPanel or LPCP), or with an application? I'm wondering if file permissions are wrong somewhere (should be 755 for directories except for the top, and 644 for files). This is a Linux/Apache server, and not a Windows server? Did the installation appear to have loaded up all the necessary files? Do their permissions (directories and files) look normal?