<?php
function create_meta_desc() {
global $post;
if (!is_single()) { return; }
$meta = strip_tags($post->post_content);
$meta = strip_shortcodes($post->post_content);
$meta = str_replace(array("\n", "\r", "\t"), ' ', $meta);
$meta = substr($meta, 0, 125);
echo "<meta name='description' content='$meta' />";
}
add_action('wp_head', 'create_meta_desc');
?>