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.
?>

Radically overhauled in September 2017. Tested and working perfectly.

More amendments. Not working that perfectly. I found using the strpos thing wasn't working that well, mainly because if the strpos position was in the middle of the final word, it would look beyond it and not find any spaces, it therefore returned false. So, I've used strrpos (note two r's) combined with a negative value offset which means it looks backwards from the offset and is guaranteed to find a space, unless of course the string is a single word longer than 150 characters. Which is possible in Wales I suppose.

Now also available as a function, right here.