Prefixing a WordPress post title with the ID of an ACF Custom Database Tables meta data row.

acfcdt-prefix-post-title-with-meta-id.php

<?php

add_action( 'acf/save_post', function ( $post_id ) {
	global $wpdb;

	// Get the meta ID from the custom DB table.
	$table_name = 'my_custom_db_table';
	$meta_id = $wpdb->get_var( $wpdb->prepare( "SELECT `id` FROM {$wpdb->prefix}{$table_name} WHERE `post_id` = %d", $post_id ) );

	// Choose a prefix format.
	$title_prefix = "$meta_id — ";

	$post = get_post( $post_id );

	// If post title is already prefixed with meta ID, don't do it again.
	if ( 0 === strpos( $post->post_title, $title_prefix ) ) {
		return;
	}

	// Update the post.
	$post->post_title = $title_prefix . $post->post_title;
	wp_update_post( $post );
} );