function addMetatags(){
	global $post;
	if(is_single() || is_page()){
		if( $post->post_excerpt != '' ){
			echo '<meta name="description" content="' . $post->post_excerpt . '" />';
		}else{
			echo '<meta name="description" content="'
				. htmlspecialchars($post->post_title)
				. '" />';
		}
		echo '<meta name="Keywords" content="';
		foreach(get_the_tags($post->ID) as $tag ){
			echo $tag->name . ", ";
		}
		echo '" />';
	}elseif(is_category()){
		echo '<meta name="description" content="'
			. htmlspecialchars(strip_tags(category_description()))
			. '" />';
		echo '<meta name="Keywords" content="default keywords" />';
	}else{
		echo '<meta name="description" content="default keywords" />';
		echo '<meta name="Keywords" content="default keywords" />';
	}
}
add_action('wp_head', 'addMetatags');