In Windows INI file (generated with the Windows installer of PHP 5.2):

; - variables_order = "GPCS"       [Performance]
;     The environment variables are not hashed into the $_ENV.  To access
;     environment variables, you can use getenv() instead.

; This directive describes the order in which PHP registers GET, POST, Cookie,
; Environment and Built-in variables (G, P, C, E & S respectively, often
; referred to as EGPCS or GPC).  Registration is done from left to right, newer
; values override older values.
variables_order = "GPCS"

As you see, variables_order does not include E so the $_ENV variable is not filled. We should have a fallback for this in eZ Publish (with the getenv function, see http://www.php.net/manual/en/function.getenv.php) , wherever we use values coming from the $_ENV array. Currently, this is only in kernel/setup/ezsetuptests.php and
lib/ezutils/classes/ezsys.php.

See http://www.php.net/manual/en/ini.core.php#ini.variables-order

Also, the documentation should mention with variables variables_order setting eZ Publish is compatible, and we even should test for it in the setup wizard.