What causes an Error 400 when running php on IIS?

When installing php on IIS, you may get an Error 400 trying to run any php scripts.

Edit php.ini and change the following setting:

; cgi.force_redirect is necessary to provide security running PHP as a CGI under
; most web servers.  Left undefined, PHP turns this on by default.  You can
; turn it off here AT YOUR OWN RISK
; **You CAN safely turn this off for IIS, in fact, you MUST.**
cgi.force_redirect = 0