
-------- constructing an HTML page -----------
* - WebApp::getHtmlPage($tpl)
    Constructs the page given as a parameter and returns
    it as an HTML string. It saves whatever is constructed
    in the current page and then resumes it again after
    constructing the requested page, so it can be called from
    anywhere in the application, e.g. from an event handler.
    This function is usefull when you want to construct a page 
    for sending it by e-mail.
    
-------- external link to an application page -----------
* - The function WebApp::external_page() returns a link that can
    be used to jump to this page of the application from any
    external page. Also the constant "SHOW_EXTERNAL_LINK", which
    is defined in 'config/const.Settings.php' makes possible to
    display the external link to the end of the page.

    The external links are useful e.g. for generating an index
    page for search engines, so that they can access and index
    the content pages of the application.

* - The framework defies a constant UNDEFINED (which has the value 
    "UNDEFINED" or something else) and is used in the framework 
    and in the application whenever a variable has no value.
    E.g. if the function WebApp::getWBox("wb_id") returns UNDEFINED,
    it means that such a webbox does not exist in the application,
    or if the function Recordset::Field($fld_name) returns UNDEFINED,
    it means that such a field does not exist or it has no value, etc.

------------------- WebApp::message() ---------------------------
* - The function:  WebApp::message("msg") can be used in PHP code to
    display a message to the user after the page is loaded in browser.
    This can be used to display an error message, e.g. "Wrong username
    or password", or an information message, e.g. "Project Data Saved".
* - The function:
     WebApp::debug_msg($msg, $comment);
    is added to the framework. It makes the framework to display
    the given message after the page is rendered. The comment is
    optional. You can also print a debug message like this:
         print "$comment: $msg<br>\n";
    but the output of this statement may be mixed with the HTML
    code and sometimes it may break the page, so the first
    method is safer and cleaner. The message $msg can be a large
    message that contains tags, e.g.
    WebApp::debug_msg($tpl->toHtmlTable()); 
    WebApp::debug_msg($rs->toHtmlTable(), "The content of recordset");
    WebApp::debug_msg($tplVars->toHtmlTable(), 
            "The content of varstack at this point");
    WebApp::debug_msg($webPage->template_list(),
                "All the templates parsed up to this point");
        etc.
-----------------------------------------------------------------
* $webPage->append_to_head($line);
  $webPage->append_to_body($line);

  These functions can be used to add more lines to the content
  of the <head> and <body> elements of a web page. The first
  one (that appends to <head>) cannot be used inside onRender(),
  because the <head> of the page has been already sent to the 
  browser.
-----------------------------------------------------------------
--------------------------------------------------------------------
* - define("VAR_NOT_FOUND", "{var_name}");
    is added to 'config/const.Settings.php'

    This constant sets the format of the error message that is displayed
    when a {{variable}} is not found. 'var_name' is replaced by
    the actual variable name. Examples: "'var_name' is not defined",
    "", "undefined", etc. It cannot contain "{{var_name}}" inside.

---------------- NULL values -----------------------------------------
* - For a DB value that was NULL, the framework used to return UNDEFINED
    and, as a result, in the page was displayed {var_name}.
    Now, the constant NULL_VALUE is returned, which can be specified
    in the file 'config/const.Settings.php'. It can be "", "NULL", NULL, etc.

--------------------------------------------------------------------
* - config/const.Debug.php: define("SHOW_ERROR_MESSAGES", true);

    If this constant is 'false' than the framework suppresses
    all the error and warning messages (that are generated by the
    framework). Usually it should be 'true' during the development and
    should be switched to 'false' when the application is deployed, 
    in order not to confuse the users of the application with error 
    messages that have no meaning for them.

--------------------------------------------------------------------
* - define("DISPLAY_CGI_VARS", false); (at 'config/const.Options.php')
    When this constant is true, then the CGI vars are displayed
    at the URL window of the browser. See also SHOW_EXTERNAL_LINK
    at const.Debug.php.

