在PHP中使用方法后，“：？”是什么意思？ [重复]

Question

This question already has an answer here:

• php method argument type hinting with question mark (?type) 2 answers
• Reference — What does this symbol mean in PHP? 18 answers

I just saw this in a Symfony 4 application, and I can't find nowhere what it means

  public function findOneBySomeField($value): ?Article
    {
        return $this->createQueryBuilder('a')
            ->andWhere('a.exampleField = :val')
            ->setParameter('val', $value)
            ->getQuery()
            ->getOneOrNullResult()
        ;
    }

I know that, now with PHP 7 you can define the expected type of the returned value with ":int $val", but here, what does the ? symbol means ?

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Answer 1

This is a new feature as of PHP 7.1. See the explanation here

Type declarations for parameters and return values can now be marked as nullable by prefixing the type name with a question mark. This signifies that as well as the specified type, NULL can be passed as an argument, or returned as a value, respectively.

This means the expected output of your function will be either an Instance of the class Article or it is NULL.