How to combine text and variables in new variable or msgbox output?

Here's what I have tried. The intended output is in a comment. I'm trying to create a source directory path using multiple variables and text. Thanks for any help.

#include <GUIConstants.au3>

#forcedef $system_environment
#forcedef $major_release
#forcedef $minor_release

call("major_release")
call("minor_release")
call("system_environment")
call("displaysourcepath")

Func major_release()
$major_release = InputBox("Enter the Major (.0) release #", "Enter the major release #")
Sleep(500)
MsgBox(4, "Is this the correct Major (.0) release #?", $major_release)
EndFunc   ;==>major_release

Func minor_release()
$minor_release = InputBox("Enter the Minor release #", "Enter the minor release #")
Sleep(500)
MsgBox(4, "Is this the correct Minor release #?", $minor_release)
EndFunc   ;==>minor_release

Func system_environment()
$system_environment = InputBox("Enter the system_environment", "Enter the system_environment")
Sleep(500)
MsgBox(4, "Is this the correct system_environment?", $system_environment)
EndFunc   ;==>system_environment

Func displaysourcepath()
$sourcepath = string("\\srv100\Source\Installation\" & $major_release & "\" & $minor_release & "." & $system_environment)
msgbox(4, "Is this the correct Source directory PATH?", $sourcepath)
;supposed to come out \\srv100\source\installation\12.1.5.0\12.1.5.6.uat
EndFunc