Custom page with content from API response

I’m trying to make a custom page created with hook_menu() and in the callback function, I need to get a variable with HTML code which is returned by a library I’m using. I tried getting it with page_callback_function($html) but page callback functions get variables from hook_menu arguments. How can I achieve this?

Sample code:

function mymodule_menu() {
$items = array();
$items['get_html_from_api/'] = array(
    'title' => 'remote HTML',
    'page callback' => 'mymodule_page_callback',
    'access callback' => TRUE,
    'access arguments' => array('access content'),
    'type' => MENU_CALLBACK,
);
return $items;
}
$html = getHTML();

function mymodule_page_callback($html) {
if ($html == NULL) {
    return t('No HMTL was sent. Error.');
}

$output = $html;
return $output;
}
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