How I add "private functions" to a pest test file.

1. TestCase.php

<?php

#
# 1. Add the Macroable trait to your base test.
#

namespace Tests;

use Illuminate\Foundation\Testing\TestCase as BaseTestCase;
use Illuminate\Support\Traits\Macroable;
...

abstract class TestCase extends BaseTestCase
{
    ...
    use Macroable;
  
    ...
}

2. helpers.php

<?php

#
# 2. Add these global helpers (just for a neat developer experience)
#

use Illuminate\Support\Arr;
use Illuminate\Support\Traits\Macroable;

function private_function(string $name, callable $callback)
{
    macro($name, $callback, $frame = 2);
}

function macro(string $name, callable $callback, int $frame = 1)
{
    $class = Arr::get(debug_backtrace(DEBUG_BACKTRACE_PROVIDE_OBJECT, $frame + 1), "{$frame}.class", '');

    $macroable = class_exists($class) && collect()
        ->wrap($class)
        ->merge(class_parents($class))
        ->map(fn (string $class) => in_array(Macroable::class, class_uses($class)))
        ->contains(true);

    throw_unless($macroable, new Exception('You must call macro from a macroable class.'));

    $class::macro($name, $callback);
}

3. ExampleTest.php

<?php

#
# 3. I use my `private_function` helper to show that this test file has a function called input.
#    You could also do `macro('input', fn)` or `$this->macro('input', fn)`. I just prefer 
#    `private_function('input', fn)` for clear intent that you can call `$this->input(...)`.
#

namespace Tests\Feature;

beforeEach(function () {
    private_function('input', function (array $input = []) {
        return array_merge([
            'name' => 'Happy Path Inputs',
            'email' => 'happy@path.inputs',
        ], $input);
    });
});

test('example', function () {
    $response = $this->post('/example', $this->input([
        'email' => 'this@works.com',
    ]));
  
    $response->assertSuccessful();
    expect(Example::count())->toBe(1);
    $example = Example::first();
    expect($example->name)->toBe('Happy Path Inputs');
    expect($example->email)->toBe('this@works.com');
});