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

wp_meta_description_function.php

<?php
// Generating a meta description using standard WP components
function meta_description( $char_limit ) {
		$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.
		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
						$description = substr( $excerpt_stripped, 0, $position ); // Trim up until the point of the last space.
			  } else {
						$description = $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 ) {
				    $content_trimmed = substr( $content_stripped, 0, $char_limit ); // Trim the raw content back to the character limit
				    $last_space = strrpos( $content_trimmed, ' ' ); // Find the last space in the trimmed content which could include incomplete words
						$description = substr( $content_trimmed, 0, $last_space ); // Re-trim the content to the point of the last space.
			  } else {
						$description = $content_stripped;
				}
		} else {
		    $description = $tagline; // If the page is empty we can use the tagline to prevent emptiness.
		}
		return $description;
}
?>

Meta description from page content

To use, add this code to your theme's functions.php file and then echo meta_description(155) in your theme's header file.

Use like this in a normal meta tag: <?php echo '<meta name="description" content="' . meta_description(155) . '">' ; ?>

Or like this as HTML: <meta name="description" content="<?php echo meta_description(155); ?>">

Parameter

$char_limit

(integer) (Required) How long you want the description to be.
Note: At the time of writing this it would appear 155 is a good length for SEO purposes.