php version for requirement needs be >= 7.1.0

[php-newer]

Nullable types ¶
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.

In tag 4.8.9, this feature is imported. So "php": ">=7.1.0"

For example,line 689 in SimpleHtmlDom.php