Wordpress auto Description

autoDescription.php

<?php 

function autoDescription() {
    global $post;
	if (!is_singular()) {return; }
	elseif(!empty( $post->post_excerpt)) {
		echo "<meta name='description' content='$post->post_excerpt' />";
	}
	else{
		$meta = apply_filters('the_content', $post->post_content);
		$meta = strip_tags($meta);
		$meta = strip_shortcodes($meta );
		$meta = str_replace(array("\n", "\r", "\t"), ' ', $meta);
		$meta = substr($meta, 0, 400);
		echo "<meta name='description' content='$meta' />";
	}
}
add_action('wp_head', 'create_meta_desc');