PHP5 bug?syntaxe, affichage de date lorsque son format est une variable

<?php
$mavar = 'toto';
echo "$mavar";
// toto
echo '$mavar';
// $mavar
echo $mavar;
// toto
?>

<?php
$dated = 'Ymd';
echo date($dated, time()); 
//20100327
echo date($dated); 
//20100327
?>

<?php
error_reporting(E_ALL);
ini_set( 'display_errors', 1 );
?>

<?php
$directory_tree[] = array(
'path' => $path,
'extension' => $extension,
'date lastMod' => date('$format_date', filemtime($path))
);
?>

<?php
$directory_tree[] = array(
'path' => $path,
'extension' => $extension,
'date lastMod' => date($format_date, filemtime($path))
);
?>

$format_date = 'c'; //// Date au format ISO 8601 (ajouté en PHP 5)	2010-01-15T23:59:03+00:00