Get an excerpt from a specific post ID or post object with a variable excerpt length

functions.php

<?php
/**
 * Get the excerpt of a specific post ID or object
 * 
 * @author Pippin Williamson
 * @link http://goo.gl/lhtZD
 * @param object/int $post The ID or object of the post to get the excerpt of
 * @param int $length The length of the excerpt in words
 * @param string $tags The allowed HTML tags. These will not be stripped out
 * @param string $extra Text to append to the end of the excerpt
 */
function iweb_get_excerpt_by_id( $post, $length = 10, $tags = '<a><em><strong>', $extra = '&hellip;' ) {
	if ( is_numeric( $post ) ) {
		$post = get_post( $post );
	} elseif( ! is_object( $post ) ) {
		return false;
	}
	
	if ( has_excerpt( $post->ID ) ) {
		$the_excerpt = $post->post_excerpt;
		return apply_filters( 'the_content', $the_excerpt );
	} else {
		$the_excerpt = $post->post_content;
	}
	
	$the_excerpt = strip_shortcodes( strip_tags( $the_excerpt, $tags ) );
	$the_excerpt = preg_split( '/\b/', $the_excerpt, $length * 2 + 1 );
	$excerpt_waste = array_pop( $the_excerpt );
	$the_excerpt = implode( $the_excerpt );
	$the_excerpt .= $extra;
	
	return apply_filters( 'the_content', $the_excerpt );
}