Get an auto generated post excerpt, or a manual excerpt if one has been set.

functions.php

<?php
// Make sure to prefix the function if you do not use a namespace.
// namespace TimJensen\HelperFunctions;

/**
 * Returns an auto generated post excerpt, or a manual excerpt if one has been set.
 *
 * @version 1.2.1
 *
 * @param int   $post_id         Required. Post ID.
 * @param mixed $excerpt_length  Optional. Length of the automatic except.
 *                               Does not affect the manual excerpt length.  Defaults to 55.
 * @param mixed $excerpt_more    Optional. Text that appends the excerpt.  Defaults to '...'.
 * @param bool  $more_link       Optional. Includes a link to the singular post.  Defaults to true.
 *
 * @return string
 */
function get_post_excerpt( $post_id, $excerpt_length = null, $excerpt_more = null, $more_link = true ) {

	$post = get_post( $post_id );

	if ( null === $excerpt_length ) {

		$excerpt_length = apply_filters( 'excerpt_length', 55 );

	}

	if ( $post->post_excerpt ) {

		$excerpt = $post->post_excerpt;

	} else {

		$excerpt = strip_shortcodes( $post->post_content );
		$excerpt = apply_filters( 'the_content', $excerpt );
		$excerpt = str_replace( ']]>', ']]&gt;', $excerpt );

		$original_word_count = str_word_count( strip_tags( $excerpt ) );

		$excerpt = wp_trim_words( $excerpt, $excerpt_length, '' );

		$is_trimmed_excerpt = str_word_count( $excerpt ) < $original_word_count;

	}

	if ( null === $excerpt_more ) {

		$excerpt_more = apply_filters( 'excerpt_more', '&hellip;' );

	}

	// Remove the excerpt more (i.e., '...') when there is no more text to show.
	$excerpt_more = empty( $is_trimmed_excerpt ) ? '' : $excerpt_more;

	$excerpt = apply_filters( 'get_post_excerpt_excerpt', $excerpt . $excerpt_more );

	if ( false === $more_link ) {

		return $excerpt;

	} else {

		$more_link = sprintf( '<a href="%s" class="more-link read-more-link">%s</a>',
			get_the_permalink( $post->ID ),
			apply_filters( 'get_post_excerpt_read_more_text', 'Read More' )
		);

	}

	return $excerpt . apply_filters( 'get_post_excerpt_read_more_link', $more_link );
}