shortcode for related post in wp - php

Question

I’m using bellow function for related post shortcode and I have a question about it. It’s supposed to display posts with related tag (I don't why it shows just random posts) but I want to use something like [rps tag=google] and the function just return a post has a tag as “google”, I mean nothing related to the current post tags. How can I do that?
This is the code:
add_shortcode('rps', 'fphp_get_related_posts');
function fphp_get_related_posts() {
$reset_post = $post;
global $post;
$post_tags = wp_get_post_tags($post->ID);
if ($post_tags) {
$post_tag_ids = array();
foreach($post_tags as $post_tag) $post_tag_ids[] = $post_tag->term_id;
$args=array(
'tag__in' => $post_tag_ids,
'post__not_in' => array($post->ID),
'posts_per_page' => 1,
'orderby' => 'rand',
);
$related_query = new wp_query( $args );
if (intval($related_query->post_count) === 0) return '';
$html = '<div class="rps"><ul><h3>Also read:</h3>';
$related_query->the_post();
$html .= '<li style="width:250px">';
$html .= '<div class="relatedthumb"><a rel="external" href="'. get_the_permalink(). '">';
$html .= get_the_title() . '</a>';
$html .= '</div></li>';
}
$post = $reset_post;
wp_reset_query();
$html .= '</ul></div>';
return $html;
}

Answer 1

If you want to show given tag post, then no need to use wp_get_post_tags(), because its return multi tags. try this code,
function fphp_get_related_posts( $atts ) {
$atts = shortcode_atts( [
'tag' => '',
], $atts );
$args = [ 'posts_per_page' => 1, 'tag' => $atts['tag'], 'orderby' => 'rand' ];
$query = new WP_Query( $args );
if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post();
ob_start(); ?>
<!-- your html here -->
<?php endwhile; endif; wp_reset_postdata();
return ob_get_clean();
}
add_shortcode( 'rps','fphp_get_related_posts' );