WordPress | Use post/page excerpt to generate meta description for SEO

header-excerpt-description.php

<?php
$tagline = get_bloginfo ( 'description' );
$post_object = get_post();
$excerpt = $post_object->post_excerpt; // Get the raw excerpt, warts (tags) and all.
$content = $post_object->post_content; // Get the raw content.
$char_limit = 150; // Set the character length to 150, best practice for SEO.
echo '<meta name="description" content="';
if ( !empty( $excerpt ) ) { // If there is an excerpt lets use it to generate a meta description
	$excerpt_stripped = strip_tags( $excerpt ); // Remove any tags using the PHP function strip_tags.
	$excerpt_length = strlen( $excerpt_stripped ); // Now lets count the characters
	if ( $excerpt_length > $char_limit ) { // Now work out if we need to trim the character length.
		$offset = $char_limit - $excerpt_length; // This gives us a negative value. 
		$position = strrpos( $excerpt_stripped, ' ', $offset ); // This starts looking for a space backwards from the offset.
		echo substr( $excerpt_stripped, 0, $position ); // Trim up until the point of the last space.
    } else {
		echo $excerpt_stripped; // The excerpt must be less than the char_limit so lets just print it.
	}
} elseif( !empty( $content ) ) {
	// If no excerpt exists we use the content, note the use of get_post as we are outside the loop.
	$content_stripped = strip_tags( $content );
	$content_length = strlen( $content_stripped );
	if ( $content_length > $char_limit ) {
        $offset = $char_limit - $content_length;
		$position = strrpos( $content_stripped, ' ', $offset );
		echo substr( $content_stripped, 0, $position );
    } else {
		echo $content_stripped;
	}
} else {
    echo $tagline; // If the page is empty we can use the tagline to prevent emptiness.
}
echo '">' . PHP_EOL; // close meta description.
?>

Now also available as a function, right here.