If you see an error "Warning: session_start..."
There is a problem with your PHP server that will prevent the CAPTCHA from working with PHP sessions. Sometimes PHP session do not work because of a file permissions problem. The solution is to make a trouble ticket with your web host, send them a URL link to this page so they can see the error and fix it. Alternatively, you can enable the setting "Use CAPTCHA without PHP Session", then temporary files will be used for storing the CAPTCHA phrase. This allows the CAPTCHA to function without using PHP Sessions. You can find this setting on the contact form admin settings page.