Use of undefined constant JSON_HEX_TAG I'm running PHP 5.3.3, which the docs say is supported, but if I try to go to the main admin page or go to add a field, I'm getting an error: 
    Use of undefined constant JSON_HEX_TAG - assumed 'JSON_HEX_TAG'

Everything I'm finding online points to this being an outdated PHP issue, but thought I would check since I am running a supported version of PHP. 

Thanks!
You're likely missing the PHP JSON extension, which is a requirement to run Craft.