PHP functions attached with ":" and "?" , what does it do?

Question

I found out a php file inside which it had a function like below:

public function getCharset(): ?string
{
    return $this->charset;
}

I want to know what is : ?string doing here.

Answer 1

This is known as a nullable type, and is introduced in PHP 7.1:

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.

Essentially, the function can return either the specified type or null. If it would return a different type, an error is thrown:

function answer(): ?int  {
    return null; // ok
}

function answer(): ?int  {
    return 42; // ok
}

function answer(): ?int {
    return new stdclass(); // error
}