Simple field example

gistfile1.php

<?php

// Of course you also need your Doctrine Annotations
class Example
{
    protected $firstName;
    protected $lastName;
    protected $username;

    public function setFirstName($firstName)
    {
        $this->username = $firstName . "." . $this->lastName;
        $this->firstName = $firstName;
    }

    public function setLastName($lastName)
    {
        $this->username = $this->firstName . "." . $lastName;
        $this->lastName = $lastName;
    }

    // getters as always
}