When PHP starts up, it picks up initial values from a file called php.ini. Under Mac OS X, the file is not mandatory (afaik) ... but  if you do need one, place it (or look for it) under
         /private/etc/php.ini

Sample php.ini files come with the distibutions ... if you're setting one up from scratch and want to test out its presence, try adding / changing the line
        post_max_size = 2M
which controls the maximum amount of data a user cn submit via the post method, and look at the current setting via a php page including
        <?php phpinfo(); ?>
(remembering to restart or otherwise kick your web server once you've edited the config file!)

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