#WordPress #ACF Get posts which includes same value of relationship field

gistfile1.php

<?php
// get current post
global $post;
// get custom field data.
// "custom_field_key" is custom field key (ACF relationship field), return value is array of post IDs
$relations = $post->custom_field_key;
// 

$meta_array = array();
foreach ((array) $relations as $relation){
	$meta_array[] = array(
		'key' => 'custom_field_key',
		'value' => '"' . $relation . '"',
		'compare' => 'LIKE'
	);
}

if (count($meta_array) > 1) $meta_array['relation'] = 'OR';

$posts = get_posts(array(
	'post_type' => 'post',
	'meta_query' => $meta_array
));

foreach ($posts as $post) {
	setup_postdata($post);
	?>
	<h1><?php the_title(); ?></h1>
	<?php
}