Foo.php

<?php
class Foo
{
  public function doStuff($i) {
    if ($i < 10) {
      return "{$i}";
    } else {
      return $this->protectedStuff($i);
    }
  }
  
  protected function protectedStuff($i) {
    return "<strong>{$i}</strong>";
  }
}

FooTest.php

<?php
class FooTest 
{
  public function fooProvider() {
    return [
      [-5, "-5"],
      [0, "0"],
      [1, "1"],
      [9, "9"],
      [10, "<strong>10</strong>"],
      [20, "<strong>20</strong>"],
      [30, "<strong>30</strong>"],
    ];
  }
  
  /**
   * @dataProvider fooProvider
   **/
  public function testStuff($input, $expectedOutput) {
    $foo = new Foo();
    
    $this->assertEquals($expectedOutput, $foo->doStuff($input));
  }
}