The print $content statement in page.tpl.php . I want to alter it but I can't figure out what/where is the source of the $content variable in page.tpl.php file.
I'd appreciate any help. Thanks.
the drupal version is 6,
chx's answer is correct. This is just a longer explanation.
$content depends on the URL or more precisely the region that calls the variable. The URL could be something like node/10, taxonomy/term/1, etc. Each of these paths is associated with menu entry which has a callback function that generates the value of $content.
Take a look at the API docs for more info. http://api.drupal.org/api/drupal/includes--common.inc/function/drupal_get_content/6