Visitor pattern implementation in PHP

visitor.php

<?php

interface EntityInterface
{
    /**
     * @param VisitorInterface $visitor
     */
    public function accept(VisitorInterface $visitor);
}

interface VisitorInterface
{
    /**
     * @param EntityInterface $entity
     */
    public function visit(EntityInterface $entity);
}

abstract class AbstractEmployee implements EntityInterface
{
    /**
     * @var string
     */
    private $fullName;

    /**
     * @var float
     */
    private $salary;

    /**
     * @param string $fullName
     */
    public function __construct(string $fullName)
    {
        $this->fullName = $fullName;
    }

    /**
     * @return int
     */
    abstract public function getWorkingHours();

    /**
     * @return float
     */
    abstract public function getHourlyRate();

    /**
     * @return string
     */
    public function getFullName() : string
    {
        return $this->fullName;
    }

    /**
     * @return float
     */
    public function getSalary() : float
    {
        return $this->salary;
    }

    /**
     * @param float $salary
     *
     * @return Employee
     */
    public function setSalary(float $salary) : self
    {
        $this->salary = $salary;

        return $this;
    }

    /**
     * @return string
     */
    public function __toString() : string
    {
        return sprintf('%s earns $%f', $this->getFullName(), $this->getSalary());
    }
}

class Developer extends AbstractEmployee
{
    /**
     * {@inheritdoc}
     */
    public function getHourlyRate() : float
    {
        return 34.56;
    }

    /**
     * {@inheritdoc}
     */
    public function getWorkingHours() : int
    {
        return 170;
    }

    /**
     * {@inheritdoc}
     */
    public function accept(VisitorInterface $visitor)
    {
        $visitor->visit($this);
    }
}

class Boss extends AbstractEmployee
{
    /**
     * {@inheritdoc}
     */
    public function getWorkingHours() : int
    {
        return 80;
    }

    /**
     * {@inheritdoc}
     */
    public function getHourlyRate() : float
    {
        return 88.55;
    }

    /**
     * @return float
     */
    public function getBonus() : float
    {
        return 325;
    }

    /**
     * {@inheritdoc}
     */
    public function accept(VisitorInterface $visitor)
    {
        $visitor->visit($this);
    }
}

class DeveloperSalaryVisitor implements VisitorInterface
{
    /**
     * {@inheritdoc}
     */
    public function visit(EntityInterface $developer)
    {
        $salary = $developer->getWorkingHours() * $developer->getHourlyRate();
        $developer->setSalary($salary);
    }
}

class BossSalaryVisitor implements VisitorInterface
{
    /**
     * {@inheritdoc}
     */
    public function visit(EntityInterface $boss)
    {
        $salary = ($boss->getWorkingHours() * $boss->getHourlyRate()) + $boss->getBonus();
        $boss->setSalary($salary);
    }
}

$developer = new Developer('Pera Zdera');
$boss = new Boss('Milojko Pantic');

$developer->accept(new DeveloperSalaryVisitor());
$boss->accept(new BossSalaryVisitor());

echo sprintf('%s', $developer);
echo "\n";
echo sprintf('%s', $boss);