If I'm right, that would also happen on other OSs if ext/soap is built as shared library.

Nope, has nothing to do with shared libs – Windows specific issue.

The problem is that SoapServer expects "wsdl" as query string to deliver the WSDL. However, if a query string doesn't contain an equals sign, command line options are ignored on Windows. So either hack-around by making SoapServer more deliberate:

 ext/soap/soap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/ext/soap/soap.c b/ext/soap/soap.c
index 48a7fc8885..d7dfc4ecd5 100644
--- a/ext/soap/soap.c
+++ b/ext/soap/soap.c
@@ -1297,7 +1297,8 @@ PHP_METHOD(SoapServer, handle)
 	if (SG(request_info).request_method &&
 	 strcmp(SG(request_info).request_method, "GET") == 0 &&
 	 SG(request_info).query_string &&
-	 stricmp(SG(request_info).query_string, "wsdl") == 0) {
+	 (stricmp(SG(request_info).query_string, "wsdl") == 0 ||
+	 stricmp(SG(request_info).query_string, "wsdl=") == 0)) {
 
 		if (service->sdl) {
 /*

or apply a proper fix for the tests, namely to spawn a php-cgi process with the command line options, and then send a CGI request and verify the response. Certainly possible, but I'm not sure it's worth the effort.

Originally posted by @cmb69 in #17432 (comment)