Trying to auto generate Meta Description.

I have added the following code to my functions.php

function gen_meta_desc() {
global $post;

if (!is_single()) {
    return;
}

//print_r($post);
$meta = strip_tags($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', 'gen_meta_desc');

But how do I get the description to output in the header?

I have tried adding <meta name='description' content='$meta' /> in the header but it doesn’t work.

What do I do?