Get the most relevant image in WordPress

relevant_image.php

<?php
/**
* Returns the attachment id for the most relevant image of the post
* @author Eric Wennerberg
* @package Flavour
*/
function get_relevant_image() {
	if(has_post_thumbnail()) {
		return get_the_featured_image_id();
	} else {
		return get_the_first_image();
	}
}


/**
* Returns relevant image source
* @author Eric Wennerberg
* @package Flavour
* @param bool | str $size
*/
function get_relevant_image_src($size = false) {
	if(!$size):
		$imageArray = wp_get_attachment_image_src(get_relevant_image());
	else:
		$imageArray = wp_get_attachment_image_src(get_relevant_image(), $size);
	endif;
	return $imageArray[0];
}

/**
* Returns the attachment id of the featured image
* @author Eric Wennerberg
* @package Flavour
* @return int 
*/
function get_the_featured_image_id() {
	return get_post_thumbnail_id(get_the_ID());
}


/**
* Returns the post's first image's id
* @author Eric Wennerberg
* @package Flavour
* @return int
*/
function get_the_first_image() {
	$output = preg_match_all('/wp-image-([0-9]*)/i', get_the_content(), $matches);
	if(empty($matches[1])) {
		return false;
	}
	return $matches[1][0];
}
?>