self::assertEmailSubjectContains($email, 'This is the subject');

<?php

declare(strict_types=1);

namespace App\Tests\Constraint;

use PHPUnit\Framework\Constraint\Constraint;
use Symfony\Component\Mime\Email;

final class EmailSubjectContains extends Constraint
{
    private string $expectedText;

    public function __construct(string $expectedText)
    {
        $this->expectedText = $expectedText;
    }

    public function toString(): string
    {
        return sprintf('contains "%s"', $this->expectedText);
    }

    protected function matches($other): bool
    {
        if (!$other instanceof Email) {
            throw new \LogicException('Can only test a message subject on an Email instance');
        }

        return str_contains((string) $other->getSubject(), $this->expectedText);
    }

    protected function failureDescription($other): string
    {
        $message = 'the email subject ' . $this->toString();
        if ($other instanceof Email) {
            $message .= sprintf('. The subject was: "%s"', $other->getSubject() ?? '<empty>');
        }

        return $message;
    }
}