Single Responsibility Principle (SRP).

SingleResponsibility.php

<?php

// Bad demo
$user = new User();

$user->create([]);

class User
{
    public function create(array $data)
    {
        try {
            // ...save user to datebase
        } catch (DatabaseException $e) {
            $this->logError($e->getMessage());
        }
    }

    public function logError()
    {
        // ...write error to file
    }
}

// Good demo
$logger = new Logger();
$user = new User($logger);

$user->create([]);

class User
{
    protected $logger;

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

    public function create(array $data)
    {
        try {
            // ...save user to datebase
        } catch (DatabaseException $e) {
            $this->logger->writeToFile($e->getMessage());
        }
    }

    public function logError()
    {
        // ...write error to file
    }
}

class Logger
{

    public function writeToFile($message)
    {
        // ...write error to file
    }
}