String performance tuning?

Hi, I wonder if there is any performance tuning newsgroup/faq/whatever related to PHP. I searchead to no avail... Anyway, my current mind boggler is: we are accumulating a large (~400k) XML response in a string before sending it out by adding little pieces a time (like 10-20 chars). Now this results in very bad performance -- most probably due to the fact that the string is enlarged/re-allocated thousands of times. I would love to pre-allocate a big buffer for the string so that performance gets better (something like SetLength() in Delphi). How can I do this? Xuse me if the answer is much too trivial, I'm a newbie in PHPland :-) Thanks for any suggestion, Peter