FizzBuzz.php

<?php
class FizzBuzz {
  function toString($number) {
    $modThreeIsZero = $number % 3 == 0;
    $modFiveIsZero  = $number % 5 == 0;

    $fizzWhen = ['', 'Fizz'];
    $numberUnless = [$number, ''];
    $buzzWhen = [$numberUnless[$modThreeIsZero], 'Buzz'];

    return $fizzWhen[$modThreeIsZero] . $buzzWhen[$modFiveIsZero];
  }
}

FizzBuzzTest.php

<?php
require_once 'FizzBuzz.php';

class FizzBuzzTest extends \PHPUnit_Framework_TestCase {
  private $fizzbuzz;

  function setUp() {
    $this->fizzbuzz = new FizzBuzz();
  }

  function testFizz() {
    $result = $this->fizzbuzz->toString(3);
    $this->assertEquals('Fizz', $result);
  }

  function testBuzz() {
    $result = $this->fizzbuzz->toString(5);
    $this->assertEquals('Buzz', $result);
  }

  function testFizzBuzz() {
    $result = $this->fizzbuzz->toString(15);
    $this->assertEquals('FizzBuzz', $result);
  }

  function testAnother() {
    $result = $this->fizzbuzz->toString(1);
    $this->assertEquals('1', $result);
  }
}

ShortFizzBuzz.php

@['Fizz'][$n % 3] . [@[$n][$n % 3 == 0], 'Buzz'][$n % 5 == 0];

Clever!