wp-add-meta-description.php

<?php
function wp24h_add_meta_tags() {
// homepage
if (is_home()) {
echo '<meta name="description" content="' . get_bloginfo( "description" ) . '" />' . "\n";
}
// single post page
if ( is_singular() ) {
global $post;
$meta_description = strip_tags($post->post_content);
$meta_description = strip_shortcodes($post->post_content);
$meta_description = str_replace(array("\n", "\r", "\t"), ' ', $meta_description);
$meta_description = substr($meta_description, 0, 160);
echo '<meta name="description" content="' . $meta_description . '" />' . "\n";
}
// category page
if ( is_category() ) {
$meta_description = strip_tags(category_description());
echo '<meta name="description" content="' . $meta_description . '" />' . "\n";
}
}
add_action( 'wp_head', 'wp24h_add_meta_tags');