Two identical strings, yet different lengths. Trimmed, cleansed, etc

try

var_dump(bin2hex($str1));
var_dump(bin2hex($str2));

or with

var_dump(htmlspecialchars($str1));
var_dump(htmlspecialchars($str2));

to inspect what's different, once found, paste here the different bits so we can figure out where the difference is from.

EDIT:

It's a & encoded as & notice that & <-> & are 4 chars different. what I think is going on is that you are sending a plain text emails, but entitizing the string, thus all & are becoming &.