为什么使用双冒号表示法指定动态类方法？

This is puzzling me: http://php.net/manual/en/dateinterval.format.php

public string DateInterval::format ( string $format )

The method is not static, yet it is given the double colons. What's the reasoning here?

That's nothing more than PHP authors'/developers' way of writing a method name qualified by its class. It isn't related to the static/dynamic nature of a method.

You'll spot this notation in PHP's errors as well. For example,

$o = new stdClass;
$o->method();

produces this error:

Fatal error: Call to undefined method stdClass::method()