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Use ZF2 classes in your app
<?php
ini_set('include_path', '/usr/local/zendsvr6/share/ZendFramework2/library');
require_once 'Zend/Loader/StandardAutoloader.php';
$loader = new Zend\Loader\StandardAutoloader(array('autoregister_zf' => true));
$loader->register();
References:
https://zend18.zendesk.com/hc/en-us/articles/222086648-Use-Framework-s-SMTP-Transport-instead-of-mail-on-IBM-i