( string str1, string str2 [, int context [, bool minimal]] )
xdiff_string_diff() makes unified diff of strings
str1 and str2.
context indicated how many lines of context you
want to include in diff result. Set minimal to
TRUE if you want to minimalize size of diff (can take a long time).
Returns string with result or FALSE if an internal error happened.
See also xdiff_file_diff().