Graham abcdef I cannot reproduce the problem on any of my mobiles .
And as xtscript is a server based code I cannot think why there would be any difference in the output
try this

var $a=<xt:browser detail="3" /> var $b=call chr $val=59 var $a=call str_replace $subject=$a;$search=(;$replace=&#40$b var $a=call str_replace $subject=$a;$search=);$replace=&#41$b print $a

the line
var $b=call chr $val=59
takes care of the semicolon
(semicolons are ignored if they are the last char on a line)