As Grunty has demonstrated, AutoIT is not typed, when a type conversion appears requested, it does its best to sort it out for you. Here is my similar example:
Opt('MustDeclareVars', 1)
Global $var1 = "1"
Global $var2 = "4"
Global $var3 = "5"
Global $var4 = $var1 & $var2 & $var3
MsgBox(1, "Concatenate", $var4)
MsgBox(1, "Concatenate+1", $var4 + 1)