Want to create macro variable

[s_lassen]

I am not quite sure what you are trying to do, but there seems to be two obvious errors in the macro code:

rc=&string1.&string2.;

This resolves to 

rc="appsrv_header('Content-type','application/vnd.openxml""formats-officedocument.spreadsheetml.sheet')";

which means that the RC variable gets the value

appsrv_header('Content-type','application/vnd.openxml"formats-officedocument.spreadsheetml.sheet')

(when the string is resolved, the double double-quote inside the double-quotes becomes a single double-quote).

I think you can get the string you want with

rc=catx(' ',&string1,&string2);

The second problem is with the assignments 

%let string3=appsrv_header('Content-disposition','attachment;;
%let string4=filename=test2.xlsx');

As the single quotes are unbalanced in the two statements, it becomes a single statement, so STRING3 becomes something like 

appsrv_header('Content-disposition','attachment;; %let string4=filename=test2.xlsx');

and STRING4 does not get defined at all. You can solve this by using the same method as for the first two variables (put the stuff in double quotes and use the CATX function), but if that will make your whole code work as intended, I have no idea.