echo php in <?php wp_list_pages( $args ); ?>

Hi,

I have a WordPress site with a lot of pages. I’m loading this in the sidebar:

<?php
if($post->post_parent) {
$children = wp_list_pages("depth=2&&title_li=&child_of=".$post->post_parent."&echo=0");
$titlenamer = get_the_title($post->post_parent);
}

else {
$children = wp_list_pages("depth=2&title_li=&child_of=".$post->ID."&echo=0");
$titlenamer = get_the_title($post->ID);
}
if ($children) { ?>

You see, to keep the overview, I’m using the ‘&depth=2‘.
Now I want to do something and I’ll try to explain this.

I want to echo a code what I’ve written in PHP, in the code:
$children = wp_list_pages("depth=2&title_li=&child_of=".$post->ID."&echo=0");

The code what I want to echo is:
<?php $exclude = ''; if($_GET['page_id'] == 33 ) $exclude='&exclude=17,38'; ?>

So, to echo the code, I have to put the code
<?php echo $exclude;?>
somewhere in the ‘&children‘ code.

Something like this:
$children = wp_list_pages("depth=2<?php echo $exclude;?>&title_li=&child_of=".$post->post_parent."&echo=0");

But, as I had expected, this doesn’t work.

My question: how can I echo PHP in the WordPress code? Of course I know I can use the ‘exclude’ code in the &children, but I want to exclude some pages ONLY when I load 1 page (when I load Page ID 33).

Thanks for helping!