WordPress: Find the first image on a page in this order: Feature image, oembed image, image in post

class.first-page-image.php

<?php
class First_Page_Image {

	private static $instance;
	private static $post_ID = 0;

	public static function instance() {
		if ( self::$instance ) {
			return self::$instance;
		}
		self::$instance = new self();
		return self::$instance;
	}

	private function __construct() {
		//add_image_size( 'opengraph-picture', 640, 480, true );
	}

	/**
	 * Find the first image on a page
	 * Priority:
	 *     1) Feature image
	 *     2) Oembed image
	 *     3) Image in post
	 *
	 * @param  int $post_id         The post ID
	 * @param  string $post_content The Post content
	 * @param  string $image_size   Optional image size, default is 'thumbnail' 
	 * @return array               image data, array(url,width,height, bool resized), see https://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src#Return_Value
	 */
	static function get_first_image_on_page($post_id, $post_content, $image_size = 'thumbnail') {

		$image_data = array();
		if (has_post_thumbnail($post_id)) {
			if (false !== ($image_data = wp_get_attachment_image_src(get_post_thumbnail_id($post_id, 'thumbnail'),$image_size))) {
				return $image_data;
			}
		} elseif (false !== ( $oembed_url = self::get_first_oembed_url($post_content)) ) {
            		$oembed_data = get_transient( md5($oembed_url) );
            		if ( empty( $oembed_data ) ) {
				$oembed_url_host = parse_url($oembed_url, PHP_URL_HOST);
				$oembed_content_url = sprintf("%s://%s/oembed?format=json&url=%s",
								(is_ssl()) ? 'https' : 'http',
								$oembed_url_host,
								$oembed_url
				);
				$response = wp_remote_request( $oembed_content_url, array(
				    'method'    => 'GET',
				    'timeout'   => 5,
				    'sslverify' => false,
				));
				if ( ! is_wp_error( $response ) ) {
				    $oembed_data = json_decode( wp_remote_retrieve_body($response), true);
				    set_transient( md5($oembed_url), $oembed_data, DAY_IN_SECONDS ); // https://codex.wordpress.org/Transients_API#Using_Time_Constants
				}
			}

			if (false !== $oembed_data) {
				$image_data = array(
					$oembed_data['thumbnail_url'],
					$oembed_data['width'],
					$oembed_data['height'],
					0
				);
			}
		} else {
			/**
			 * search through content for images
			 */
			$doc = new DOMDocument();

			// start libxml error managent
			// modify state
			$libxml_previous_state = libxml_use_internal_errors(true);
			@$doc->loadHTML($post_content);
			// handle errors
			libxml_clear_errors();
			// restore
			libxml_use_internal_errors($libxml_previous_state);
			// end libxml error management
			
			$images = $doc->getElementsByTagName('img');
			foreach ($images as $img) {
		    		$url =  $img->getAttribute('src');
				if ( false !== ($attchment_id = self::get_attachment_id_from_url($url)) ) {
					if (false !== ($image_data = wp_get_attachment_image_src($attchment_id,$image_size))) {
						return $image_data;
					}
				}
			}
		}
		return $image_data;
	}

	/**
	 * Find the first oembed native url in a post
	 * @param  string $post_content The post content
	 * @return bool|string          return false if no url is found, otherwise return the url
	 */
	static function get_first_oembed_url($post_content) {
		require_once( ABSPATH . WPINC . '/class-oembed.php' );
		$wp_oembed = _wp_oembed_get_object();
		$urls = wp_extract_urls( $post_content );
		foreach( (array) $urls as $url ) {
			$oembed_url_host = parse_url($url, PHP_URL_HOST);
			$self_url_host = parse_url(get_bloginfo( 'url') , PHP_URL_HOST);
			$site_url_host = parse_url( get_site_option( 'siteurl',  $self_url_host), PHP_URL_HOST);
			if ( false === in_array($oembed_url_host, array( $self_url_host, $site_url_host) ) ) { // don't oembed yourself
				if ( false !== ($provider_url = $wp_oembed->get_provider( $url ) ) ) {
					return $url;
				}
			}
		}
		return false;
	}

	/**
	 * Find all the oembed native url in a post
	 * @param  string $post_content The post content
	 * @return array          		An array with all the urls found
	 */
	static function get_all_oembed_urls($post_content) {
	    require_once( ABSPATH . WPINC . '/class-oembed.php' );
	    $wp_oembed = _wp_oembed_get_object();
	    $urls = wp_extract_urls( $post_content );
	    $oembed_urls = array();
	    foreach( (array) $urls as $url ) {
	        if( $wp_oembed->get_provider( $url ) ) {
	            $oembed_urls[] = $url;
	        }
	    }
		return array_unique($oembed_urls);
	}

	// from https://philipnewcomer.net/2012/11/get-the-attachment-id-from-an-image-url-in-wordpress/
	private static function get_attachment_id_from_url( $attachment_url = '' ) {
		global $wpdb;
		$attachment_id = false;
		// If there is no url, return.
		if ( '' == $attachment_url )
			return;
		// Get the upload directory paths
		$upload_dir_paths = wp_upload_dir();
		// Make sure the upload path base directory exists in the attachment URL, to verify that we're working with a media library image
		if ( false !== strpos( $attachment_url, $upload_dir_paths['baseurl'] ) ) {
			// If this is the URL of an auto-generated thumbnail, get the URL of the original image
			$attachment_url = preg_replace( '/-\d+x\d+(?=\.(jpg|jpeg|png|gif)$)/i', '', $attachment_url );
			// Remove the upload path base directory from the attachment URL
			$attachment_url = str_replace( $upload_dir_paths['baseurl'] . '/', '', $attachment_url );
			// Finally, run a custom database query to get the attachment ID from the modified attachment URL
			$attachment_id = $wpdb->get_var( $wpdb->prepare( "SELECT wposts.ID FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta WHERE wposts.ID = wpostmeta.post_id AND wpostmeta.meta_key = '_wp_attached_file' AND wpostmeta.meta_value = '%s' AND wposts.post_type = 'attachment'", $attachment_url ) );
		}
		return $attachment_id;
	}


} //class First_Page_Image

funtions-example.php

<?php

require_once ('class.first-page-image.php');
//initiate the class
if ( defined( 'ABSPATH' ) ) {
	First_Page_Image::instance();
}

$image_data  = First_Page_Image::get_first_image_on_page( $post->ID, $post->post_content );
list( $image_url, $image_width, $image_height, $image_is_resized) = $image_data;