The only difference I see is the blank before the strings but I can't erase them and the length are the same so I think there is no matter of that.
Edit : I try this in condition :
"test" = "test" and It works fine so I suppose my two string are different but I know there are the same and the logger say that. I don't understand the spaces before the string. I try normalize-space but It doesn't work to.
I try string(/process_data/@string1) and the same for string2 but I didn't work to. I try to use serialize when I take the string in the xml but It didn't change anything.
But finally It works. The string1 variable is a input variable but I can retrieve this value in the process so I delete it and create an other string but not input this time. And this time it's work without serialise() or string(), I don't understand.