I think it depends upon what you are trying to do, overall.
To change it universally for php run from your domain, you would modify your php.ini file, and set the new value there.
You could also, of course, install your own PHP (see the "see also" section of either of the pages linked above for links)
You could also do it "on the fly" but this could be more complicated depending upon what you are trying to do,as you need to make sure to keep the new value set from page to page:
Note: running PHP-CGI (the default on DreamHost), you cannot change this value via .htaccess.