I'm putting this here in hopes Google will find it and save someone else the weeks of annoyance.

If you have WordPress, and the "Upgrade Plugin" or "Upgrade Wordpress" asks you for FTP credentials, then immediately returns with a blank page, check the settings that were used to compile PHP. If you do not include "--enable-ftp" in the configure step, this is exactly what will happen. Recompile PHP using that flag, and everything will start working again.