<?php
/*
Plugin Name: Display Custom Id
Description: This plugin assigns a random custom id to each registered user, you can show your custom id by inserting a short code into your posts.
Author: Mahamad Sayed
*/
// generste random customer ID
function generate_custom_id (){
	$new_id = rand(1, 1000);
	return $new_id;
}

// Assign generated ID to user meta
function assign_custom_id() {
	$custom_id = generate_custom_id();
	$meta_key= 'custom_id';

	// We need to get all users
	$wp_user_query = new WP_User_Query( array( 'role' => '' ) );
	$users = $wp_user_query->get_results();
	foreach ($users as $user)
	{
		add_user_meta( $user->id, $meta_key, $custom_id, true );
	}
}
register_activation_hook( __FILE__, 'assign_custom_id' );

function retrieve_custom_id(){
	$result = get_user_meta(get_current_user_id(), 'custom_id', true);
	echo "<h5>Your custom Id is: user-" . $result . "</h5>";
}
add_shortcode( 'getcustomid', 'retrieve_custom_id' );