Prototype of a listener cleaning up all properties of a PHPUnit class. WARNING: This didn't decrease any memory usage for me!

CleanUpPropertiesTestListener.php

<?php

declare(strict_types=1);

namespace App\Tests;

use PHPUnit\Framework\Test;
use PHPUnit\Framework\TestListener;
use PHPUnit\Framework\TestListenerDefaultImplementation;
use PHPUnit\Framework\TestSuite;

class CleanUpPropertiesTestListener implements TestListener
{
    use TestListenerDefaultImplementation;

    /**
     * The "endTestSuite" is called for every TestCase file, so we can make usage of that to clean up properties.
     */
    public function endTestSuite(TestSuite $suite): void
    {
        if (!\class_exists($suite->getName())) {
            return;
        }

        $beforeMemoryUsage = \memory_get_usage(true);

        foreach ($suite->tests() as $test) {
            $this->cleanUpProperties($test);
        }

        $suite->setTests([]);
        \gc_collect_cycles();

        echo \PHP_EOL;
        echo $suite->getName();
        echo ' (' . $this->human_filesize($beforeMemoryUsage) . ')';
        echo ' (' . $this->human_filesize(\memory_get_usage(true)) . ')';
    }

    private function human_filesize(int $bytes, int $dec = 2): string
    {
        $size = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
        $factor = \floor((\strlen((string) $bytes) - 1) / 3);
        if (0.0 === $factor) {
            $dec = 0;
        }

        return \sprintf("%.{$dec}f %s", $bytes / (1024 ** $factor), $size[$factor]);
    }

    public function cleanUpProperties(Test $test): void
    {
        $reflectionClass = new \ReflectionClass($test);
        $properties = $reflectionClass->getProperties();

        $unsetFunction = function (string $valueName, bool $isStatic): void {
            if (!$isStatic) {
                if (isset($this->{$valueName})) {
                    unset($this->{$valueName});
                }

                return;
            }

            if (isset($this::$valueName)) {
                $this::$valueName = null;
            }
        };

        foreach ($properties as $property) {
            // Ignore `PHPUnit` properties
            if (\str_starts_with($property->class, 'PHPUnit\\Framework\\')) {
                continue;
            }

            // Ignore `ProphecyTrait` properties
            if (\in_array($property->getName(), ['prophet', 'prophecyAssertionsCounted'], true)) {
                continue;
            }

            $unsetFunction->call(
                $test,
                $property->getName(),
                $property->isStatic(),
            );
        }
    }
}