Dependency Inversion Principle (DIP).

DependencyInversion.php

<?php

// Bad demo
$errorHandler = new ErrorHandler();

set_error_handler(function($errno, $errstr, $errfile, $errline) use ($errorHandler) {
    return $errorHandler->error($errno, $errstr, $errfile, $errline);
});

set_exception_handler(function($exception) use ($errorHandler) {
    return $errorHandler->exception($exception);
});

class ErrorHandler
{
    protected $logger;

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

    public function error($errno, $errstr, $errfile, $errline)
    {
        $this->logger->log('Error:' . $errstr);

        // ...display error message to the user
    }

    public function exception(Exception $exception)
    {
        $this->logger->log('Error:' . $exception->getMessage());

        // ...display error message to the user
    }
}

class Logger
{
    public function log($message)
    {
        // ...write error to file
    }
}

// Good demo
$logger = new Logger();
$errorHandler = new ErrorHandler($logger);

set_error_handler(function($errno, $errstr, $errfile, $errline) use ($errorHandler) {
    return $errorHandler->error($errno, $errstr, $errfile, $errline);
});

set_exception_handler(function($exception) use ($errorHandler) {
    return $errorHandler->exception($exception);
});

class ErrorHandler
{
    protected $logger;

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

    public function error($errno, $errstr, $errfile, $errline)
    {
        $this->logger->log('Error:' . $errstr);

        // ...display error message to the user
    }

    public function exception(Exception $exception)
    {
        $this->logger->log('Error:' . $exception->getMessage());

        // ...display error message to the user
    }
}

class Logger
{
    public function log($message)
    {
        // ...write error to file
    }
}