WordPress Plugin Expamle AMP Random Image

ppp-amp-random-image.php

<?php
/*
 * Plugin Name: PPP Random AMP Image (needs AMP plugin to work!)	
 * Description: Add random category image to amp articles without post_thumbnail. Install and activate <a href="https://de.wordpress.org/plugins/amp/">AMP Plugin</a> first
 * 
 */
 
function pppf_add_media_taxonomy() {
	
	$labels = array(
		'name'              => _x( 'Media Category', 'taxonomy general name'  ),
		'singular_name'     => _x( 'Media Category', 'taxonomy singular name' ),
		'menu_name'         => __( 'Media Categories'  ),
	);

	$args = array(
		'labels'            => $labels,
		'hierarchical'      => true,
		'show_ui'           => true,
		'show_admin_column' => true,
		'query_var'         => true,
		'show_admin_column' => true,
		'show_in_nav_menus'	=> false,
        'query_var'         => false,
        'rewrite'           => false,		
	);

	register_taxonomy( 'media-category', array( 'attachment' ), $args );
}

add_action ( 'init' , 'pppf_add_media_taxonomy' );	
 
 
function pppf_get_random_image() {
 
	$args = array(
		'post_type' => 'attachment',
		'post_status' => 'any',
		'orderby' => 'rand', // select image by random.
		'posts_per_page' => 1, // number of random image, default is 1 random image, you can change number to for example 10 to display 10 random images.
		'post_mime_type' => array('image/png', 'image/x-png', 'image/jpeg', 'image/jpg'), // image mime type (image format), you can remove some mime type or adding custom mime type.
		'tax_query' => array(
			array(
				'taxonomy' => 'media-category',
				'field' => 'slug',
				'terms' => 'random'
			)
		), 
	);
 
	$images = get_posts( $args );
 
	foreach ( $images as $image ) {
		return $image->ID;
	}
	 
	wp_reset_postdata();
 
}

function pptf_amp_modify_json_metadata( $metadata, $post ) {
	
	if  ( has_post_thumbnail( $post->ID ) ) return $metadata;
	
	$img_id 	= pppf_get_random_image();
	if ( $img_id ) {
		$img_array  = wp_get_attachment_image_src( $img_id, 'large' );
		if ( !array_key_exists( 'image', $metadata ) ) {
			$metadata['image'] = array(
				'@type' 	=> 'ImageObject',
				'url' 		=> $img_array[0],
				'width' 	=> $img_array[1],
				'height' 	=> $img_array[2]
			);
		}
	}
    return $metadata;
	
}

add_filter( 'amp_post_template_metadata', 'pptf_amp_modify_json_metadata', 10, 2 );


/*
 * if post has no thumbail use random image instead
 * theme must support post_thumbnails
 */
function pppf_post_thumbnail_html( $html ) {

	if ( empty( $html ) ) {
		$img_id 	= pppf_get_random_image();
		$img_array  = wp_get_attachment_image_src( $img_id, 'large' );
		$title 		= get_the_title($img_id);
		$html 		= sprintf('<img src="%1$s" width="%2$s" height="%3$s" alt="%4$s" />', esc_url($img_array[0]),esc_attr($img_array[1]),esc_attr($img_array[2]),esc_attr($title));
	}
	return $html;
}

add_filter( 'post_thumbnail_html', 'pppf_post_thumbnail_html' );