Sometimes when writing a test, you come across a situation where you need to set a private field that's not accessible through any existing route. For instance, I've been working with Doctrine a bit lately, and since the ID on an entity is generated automatically, it should not be possible to change it via a setter, but at the same time, we sometimes have the need to set it in a test.

Fortunately, there is a way to do that. Using PHP's reflection API, you can temporarily mark a property on an object as accessible, so as to be able to set it without either passing it to the constructor or creating a setter method that will only ever be used by the test. We first create a ReflectionClass instance from the object, then get the property. We mark it as accessible, and then set the value on the instance, as shown below:

1<?php declare(strict_types = 1);
2
3namespace Tests\Unit;
4
5use Tests\TestCase;
6use Project\Document;
7use ReflectionClass;
8
9final class DocumentTest extends TestCase
10{
11    public function testGetId()
12    {
13        $doc = new Document();
14        $reflect = new ReflectionClass($doc);
15        $id = $reflect->getProperty('id');
16        $id->setAccessible(true);
17        $id->setValue($doc, 1);
18        $this->assertEquals(1, $doc->getId());
19    }
20}

If you're likely to need this in more than one place, you may want to pull this functionality out into a trait for reuse:

1<?php declare(strict_types = 1);
2
3namespace Tests\Traits;
4
5use ReflectionClass;
6
7trait SetsPrivateProperties
8{
9    /**
10     * Sets a private property
11     *
12     * @param mixed $object
13     * @param string $property
14     * @param mixed $value
15     * @return void
16     */
17    public function setPrivateProperty($object, string $property, $value)
18    {
19        $reflect = new ReflectionClass($object);
20        $prop = $reflect->getProperty($property);
21        $prop->setAccessible(true);
22        $prop->setValue($object, $value);
23        $prop->setAccessible(false);
24    }
25}

Then your test can be simplified as follows:

1<?php declare(strict_types = 1);
2
3namespace Tests\Unit;
4
5use Tests\TestCase;
6use Project\Document;
7use Tests\Traits\SetsPrivateProperties;
8
9final class DocumentTest extends TestCase
10{
11    use SetsPrivateProperties;
12
13    public function testGetId()
14    {
15        $doc = new Document();
16        $this->setPrivateProperty($doc, 'id', 1);
17        $this->assertEquals(1, $doc->getId());
18    }
19}

While this is a slightly contrived and limited example, and this situation is quite rare, I've found it to be a useful technique under certain circumstances.