index.php

<?php

require_once('vendor/autoload.php');

use Logger\LoggerLaravel;

$logger = new LoggerLaravel();

if($logger instanceof \Logger\LoggerAbstract) {
    echo 'eh instanceof \Logger\LoggerAbstract<br>';
}

if($logger instanceof \Logger\LoggerInterface) {
    echo 'eh instanceof \Logger\LoggerInterface';
}

LoggerAbstract.php

<?php

namespace Logger;

abstract class LoggerAbstract implements LoggerInterface {
    protected $prefix = "";

    protected function getStrToLog($str) {
        return $this->prefix . " - " . $str;
    }

    public function setPrefix($prefix) {
        $this->prefix = $prefix;
    }
}

LoggerInterface.php

<?php

namespace Logger;

interface LoggerInterface {
    public function alert($error, $contextInfo = []);
}

LoggerLaravel.php

<?php

namespace Logger;

class LoggerLaravel extends LoggerAbstract {
    public function alert($error, $contextInfo = []) {
        echo 'Alert... error...';
    }
}