Trait Injection

bootstrap.php

<?php

// assumes DI container was already declared

$di->setter['BaseNamespace\Trait\Loggable']['setLogger'] = $di->lazyNew('MyAwesomePSRCompliantLoggingClass');
$di->setter['BaseNamespace\Trait\Mailable']['setMailer'] = $di->lazyNew('BaseNamespace\Service\EmailService');

// set up params for EmailService, MyAwesomePSRCompliantLoggingClass, SomeParentCLass, etc.

// advantages over constructor injection: less huge constructors of doom on, for example, controllers
// and yes, my controllers aren't aware that my container exists...their deps are all injected

// advantages over interface-based setter injection: less code in the injected classes, more consistent
// use of injected services...log() works the same on anything that pulls in loggable

// disadvantages: tighter coupling vs. interfaces (though I believe you can have a trait conform to an iface).
// slightly less obvious than constructor injection what deps are being used, though trait use's are nearby

EmailService.php

<?php

namespace BaseNamespace\Service;

// this should be an interface but I'm lazy
class EmailService
{
    /* properties */
    
    public function __construct(/* params */) {
        /* set stuff */
    }
    
    public function sendRegistrationEmail($data) {
        /* mail stuff */
    }
}

Injected.php

<?php

namespace BaseNamespace\Domain;

use BaseNamespace\Trait\Mailable;
use BaseNamespace\Trait\Loggable;

// SomeParentClass has its own constructor-injected params, but a logger and mailer aren't injected
class Injected extends SomeParentClass
{
    use Mailable, Loggable;
    
    public function someMethod($some_param) {
        $this->log('I did a thing', 'info');
        $this->getMailer()->sendRegistrationEmail($this->someParentMethod($some_param));
    }
}

Loggable.php

<?php

namespace BaseNamespace\Trait;

use Psr\Log\LoggerInterface as Logger;

trait Loggable
{
    /** @var Logger */
    protected $logger;

    public function setLogger(Logger $logger) {
        $this->logger = $logger;
    }

    public function getLogger() {
        return $this->logger;
    }

    public function log($level, $message,  $context = []) {
        if ($msg === null)
            return $this->logger;
        if ($this->logger)
            return $this->logger->msg($level, $message, $context);
        error_log(($level ?: 'debug') . ' - ' . $message . ' ' . json_encode($context));
    }
}

Mailable.php

<?php

namespace BaseNamespace\Trait;

use BaseNamespace\Service;

trait Mailable
{
    protected $mailer;

    public function setMailer(EmailService $mailer) {
        $this->mailer = $mailer;
    }

    /** @return EmailService */
    public function getMailer() {
        return $this->mailer;
    }
}