[Typo3] Pls. help. Extension, main function, fe rendering. Thks.

[mario]

Hello,

looking for help again.
I am trying to understand how to modify an extension, as to include some
kind of fe rendering.

To try to understand what I need, I define a very simple extension.
Therein the >main< function is defined as it follows. 

..
require_once(PATH_tslib."class.tslib_pibase.php");
...
function main($content,$conf)	{
			$this->conf = $conf; // Setting the TypoScript passed to this
function in $this->conf
	$this->pi_setPiVarDefaults();
	$this->pi_loadLL();	     // Loading the LOCAL_LANG values
	$this->pi_USER_INT_obj = 1;  // Disable caching
		
	$entryTable = 'tx_MY_book';
	t3lib_div::loadTCA($entryTable);
        $tcaFields =
explode(',',$GLOBALS['TCA'][$entryTable]['feInterface']['fe_admin_fieldList']);

	foreach($tcaFields as $key => $value){$mycontent .=
$key."-->".$value."<BR>";}
		return $mycontent;
	}

Now, $mycontent seems to be empty and nothing is returned on the FE.
Why?
Note, that (almost) the same code works fine within a PHPscript plugin:

<?php

require_once(PATH_tslib."class.tslib_pibase.php");

$entryTable = 'tx_bookdb_book';

t3lib_div::loadTCA($entryTable);

$tcaFields =
explode(',',$GLOBALS['TCA'][$entryTable]['feInterface']['fe_admin_fieldList']);

foreach($tcaFields as $key => $value){$mycontent .=
$key."-->".$value."<BR>";}
		echo  $mycontent;

?>

Any hint is welcome.
Thanks 
mario