No, `0` is the default value, i.e. without passing a $length, the
function behaves as if `0` was passed.

The document also states the following:
If length is specified and is zero, no elements will be removed

If zero was the default value the function would do nothing unless you passed a value into that parameter. 

When I look in my IDE however it has the following, which I believe is correct:
function array_splice (array &$input, $offset, $length = null, $replacement = null) {}

> If length is specified and is zero, no elements will be removed

You're right. Actually, the default value is `count($input)`[1].

It's definitely not NULL, though, see <https://3v4l.org/6AAFC>.

[1] <https://github.com/php/php-src/blob/PHP-7.0.11/ext/standard/array.c#L2867-L2871>

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.