Wordpress image fall back algorithm

wp_image_fallback_algorithm.php

/*
    fallback order:
    use post thumbnail(featured post) if available, otherwise
    use first attachment image if available, otherwise
    use first image link if available, otherwise
    stay blank...
*/

// inside a loop

$ID = get_the_ID();

//default is thumbnail
if($include_post_thumbnail == "false"):
	$POST_THUMBNAIL = '';
else:
	$POST_THUMBNAIL = get_the_post_thumbnail($ID, array($post_thumbnail_width, $post_thumbnail_height));
	// var_dump( $POST_THUMBNAIL );
	
	// if post thumbnail is empty, fall back to first attachment in thumb-featured size
	if( empty( $POST_THUMBNAIL ) ):
		$args = array(
		'post_type' => 'attachment',
		'numberposts' => -1,
		'post_status' => null,
		'post_parent' => $ID
		);
		$attachments = get_posts($args);
		if ($attachments):
			foreach ($attachments as $attachment):
				if(wp_attachment_is_image( $attachment->ID )):
					// echo '<a href="'. wp_get_attachment_url($attachment->ID) . '">Text Link</a>';
					$POST_THUMBNAIL = wp_get_attachment_image( $attachment->ID, 'thumb-featured' );
					// var_dump( wp_get_attachment_image( $attachment->ID, 'thumb-featured' ) );
					break;
				endif;
			endforeach;
		endif;
	endif;
	
	//if attachment is not set because using link due to import or whatever reason, parse and use first img link
	if( empty( $POST_THUMBNAIL ) ):
		// $post_content = get_post( $ID )->post_content;
		global $post;
		$post_content = $post->post_content;
		$post_content = substr( $post_content, 0, 1300 );
		// var_dump( substr( $post_content, 0, 1000 ) );
		// echo "<br><br>";
		// $post_content = '';
		// var_dump( $post );
		if (preg_match('/<img[^>]+src=[\'"]([^\'"]+)[\'"][^>]*>/i', $post_content, $matches)):
			$src = $matches[1];
			@list($width, $height) = getimagesize($src);
			$POST_THUMBNAIL = '<img src="' . $src . '" width="'.$width.'" height="'.$height.'" alt="' . get_the_title() . '" class="" />';
			// var_dump( $img );
		endif;
	endif;
endif;