A simple test class checking how a FizzBuzz function should behave.

FizzBuzzTest.php

<?php

declare(strict_types=1);

use PHPUnit\Framework\TestCase;

class FizzBuzzTest extends TestCase
{
    /**
     * @dataProvider fizzBuzzHappyCaseDataProvider
     */
    public function testFizzBuzzHappyCases(int $start, int $end, string $expectedOutput): void
    {
        self::assertEquals($expectedOutput, fizzBuzz($start, $end));
    }

    public function fizzBuzzHappyCaseDataProvider(): array
    {
        return [
            // Multiples of three become "Fizz"
            [1, 4, '1,2,Fizz,4'],
            // Multiples of five become "Buzz"
            [1,5, '1,2,Fizz,4,Buzz'],
            // Multiples of 15 become "Fizz Buzz"
            [1, 20, '1,2,Fizz,4,Buzz,Fizz,7,8,Fizz,Buzz,11,Fizz,13,14,Fizz Buzz,16,17,Fizz,19,Buzz'],
        ];
    }

    /**
     * @dataProvider fizzBuzzFaultyCaseDataProvider
     */
    public function testFizzBuzzExceptions(
        int $start,
        int $end,
        string $expectedException,
        string $expectedExceptionMessage
    ): void {
        self::expectExceptionMessage($expectedExceptionMessage);
        self::expectException($expectedException);

        fizzBuzz($start, $end);
    }

    public function fizzBuzzFaultyCaseDataProvider(): array
    {
        return [
            // Negative ranges are impossible
            [1, -10, InvalidArgumentException::class, 'Invalid range: $end (-10) must be greater than $start (1).'],
        ];
    }
}

A possible implementation:

<?php

declare(strict_types=1);

function fizzBuzz(int $start, int $end): string
{
    if ($end - $start <= 0) {
        throw new InvalidArgumentException(
            sprintf('Invalid range: $end (%d) must be greater than $start (%d).', $end, $start)
        );
    }

    // Allocate necessary memory
    $parts = array_fill($start, $end, null);

    for ($i = $start; $i <= $end; $i++) {
        $parts[$i] = $i;

        if ($i % 3 === 0) {
            $parts[$i] = 'Fizz';
        }

        if ($i % 5 === 0) {
            $parts[$i] = 'Buzz';
        }

        if ($i % 15 === 0) {
            $parts[$i] = 'Fizz Buzz';
        }
    }

    return implode(',', $parts);
}