Use `@dataProvider` to quickly pass a bunch of invalid input through your function to make sure it fails correctly.

my_test.php

<?php
use PHPUnit\Framework\TestCase;

class MyTest extends TestCase {
  /**
   * @dataProvider invalidArgumentProvider
   * @expectedException InvalidArgumentException
   */
  public function test_should_throw_on_invalid_arguments($arg) {
    my_func($arg);
  }

  public function invalidArgumentProvider() {
    return [
      [new stdClass],
      [true],
      [999]
    ];
  }
}