change META descriptions for each post?

I tried using this code, but I got an error message. Do I need to delete something first from my header.php file?

<meta name=”description” content=”<?php if (have_posts() && is_single() OR is_page()):while(have_posts()):the_post();
$out_excerpt = str_replace(array(“\r\n”, “\r”, “\n”), “”, get_the_excerpt());
echo apply_filters(‘the_excerpt_rss’, $out_excerpt);
endwhile;
elseif(is_category() OR is_tag()):
if(is_category()):
echo “Posts related to Category:
“.ucfirst(single_cat_title(“”, FALSE));
elseif(is_tag()):
echo “Posts related to Tag:
“.ucfirst(single_tag_title(“”, FALSE));
endif;
else: ?><?php bloginfo(‘description’) ?>
<?php endif; ?>” />