500 Errors are usually caused by PHP mis-configuration, have you tried changing PHP configuration file?
Have you installed PHP from WPI? --> looks like to me for the name mentioned "php55_via_fastcgi" is usually given by WPI installs.
Try this
Delete all the PHP and as well Module Mapping settings from IIS interface. Restart from scratch, also make sure you have both 32 and 64 bit versions of VC++ compiler installed. Download correct version (NTS - Non-Thread Safe) and do the manual setup.
Also, make sure you're using correct version of MS-SQL php driver, not that it's performance is bad, but with PHP it's always recommended using MySQL/MariaDB, it also gives you freedom to move to Linux platform without a hitch should you need to in future.
if you can spare sometime, Do have a look at blog post HERE, though is for IIS 8.5 on Server 2012 R2, but set up method remains the same.
