Bug: Flags needs default value

[John-Betong]

Info
I have been trying for over a week to trace a new site's "Network connection Error" from going down overnight. No errors in logs so added "strict_types=1" to all PHP app and system PHP files, also set .env -> CI_ENVIRONMENT = development and eventually managed to get some errors reported.

Describe the bug
incorrect type being passed.

CodeIgniter 4 version
CI_VERSION 4.1.1

Affected module(s)
File: ./system/HTTP/RequestTrait.php
[code]
public function fetchGlobal($method, $index = null, $filter = null, $flags = null)
{
$method = strtolower($method);

	if (! isset($this->globals[$method]))
	{
		$this->populateGlobals($method);
	}

	// Null filters cause null values to return.
	if (is_null($filter))
	{
		$filter = FILTER_DEFAULT;
	}

	// Return all values when $index is null
	if (is_null($index))
	{
		$values = [];
		foreach ($this->globals[$method] as $key => $value)
		{
			# PROBLEM 2021-03-09
			  $flags = $flags ?? 0;

			$values[$key] = is_array($value)
				? $this->fetchGlobal($method, $key, $filter, $flags)
				: filter_var($value, $filter, $flags);
		}

		return $values;
	}

[/code]

Expected behavior, and steps to reproduce if appropriate
I did not expect the web page to crash :(

Context

• OS: [Ubuntu 20.04.1]
• Web server [Apache 2]
• PHP version [8.0.3]

[MGatner]

Similar problems in:

• HTTP/RequestTrait ([UG] Update License copyright #4010)
• Validation/FormatRules (Supply Xdebug 3 mode #4011)

[paulbalandan]

Fixed by #4390