Some example code for a blog post on PMG.co

acf.php

<?php

function pmg_exampletheme_get_field($field)
{
    if (!function_exists('get_field')) {
        return null;
    }

    return get_field($field);
}

acf2.php

<?php

function pmg_exampletheme_get_field($field)
{
    if (!function_exists('get_field')) {
        throw new \LogicException('The Advanced Custom Fields Plugin is Not Loaded');
    }

    return get_field($field);
}

actions.php

<?php

// in a theme file
do_action('pmg_exampletheme_some_action', $post);


// in a plugin
add_action('pmg_exampletheme_some_action', 'pmg_exampleplugin_some_action');
function pmg_examplepluing_some_action($post)
{
    // do stuff with $post to create some nice display
}

CustomerMeta.php

<?php

namespace PMG\ExampleCustomer;

/**
 * An API class for fetching custom metadata for a given customer post type.
 */
final class CustomerMeta
{
    const WEBSITE_LINK = '_pmg_examplecustomer_websitelink';

    /**
     * The customer's post ID
     *
     * @var     int
     */
    private $postId;

    public function __construct($postId)
    {
        $this->postId = $postId;
    }

    /**
     * Get the link to the customer's home page.
     *
     * @return  string
     */
    public function getWebsiteLink()
    {
        return get_post_meta($this->postId, self::WEBSITE_LINK, true);
    }

    // ...
}

Customers.php

<?php

final class Customers
{
    private $cache = [];

    private static $ins = null;

    public function __construct()
    {
        $this->hasPlugin = class_exists('PMG\\ExampleCustomer\\CustomerMeta');
    }

    // if you want to share the same instance...
    public static function instance()
    {
        if (!self::$ins) {
            self::$ins = new self();
        }

        return self::$ins;
    }

    public function getWebsiteLink($postId)
    {
        $customer = $this->loadCustomer($postId);
        return $customer ? $customer->getWebsiteLink($postId) : null;
    }

    private function loadCustomer($postId)
    {
        if (!$this->hasPlugin) {
            return null;
        }

        if (!isset($this->cache[$postId])) {
            $this->cache[$postId] = new \PMG\ExampleCustomer\CustomerMeta($postId);
        }

        return $this->cache[$postId];
    }
}