Deprecate filter.default

There's a subtlety here regarding filter.default_flags. The RFC
stated that no separate deprecation warning will be emitted for
it, on the premise that it is only meaningful if filter.default
is set. However, it turns out that even FILTER_UNSAFE_RAW (which
is the "no-op filter") can be combined with stripping flags.
However, this is compensated by the fact that this doesn't actually
work when setting filter.default=unsafe_raw and non-default
filter.default_flags -- the flags are simply ignored in that case.
So everything works out in the end.

Part of https://wiki.php.net/rfc/deprecations_php_8_1.

Author: nikic

UPGRADING

@@ -360,6 +360,8 @@ PHP 8.1 UPGRADE NOTES
   . The FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED filters have been
     deprecated.
     RFC: https://wiki.php.net/rfc/deprecations_php_8_1
+  . The filter.default ini setting is deprecated.
+    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
 
 - Hash:
   . The mhash(), mhash_keygen_s2k(), mhash_count(), mhash_get_block_size() and

ext/filter/filter.c

@@ -107,6 +107,9 @@ static PHP_INI_MH(UpdateDefaultFilter) /* {{{ */
 	for (i = 0; i < size; ++i) {
 		if ((strcasecmp(ZSTR_VAL(new_value), filter_list[i].name) == 0)) {
 			IF_G(default_filter) = filter_list[i].id;
+			if (IF_G(default_filter) != FILTER_DEFAULT) {
+				zend_error(E_DEPRECATED, "The filter.default ini setting is deprecated");
+			}
 			return SUCCESS;
 		}
 	}

ext/filter/tests/004.phpt

@@ -4,6 +4,7 @@ GET/POST/REQUEST Test with filtered data
 filter
 --INI--
 filter.default=special_chars
+error_reporting=E_ALL&~E_DEPRECATED
 --POST--
 d="quotes"&e=\slash
 --GET--

ext/filter/tests/005.phpt

@@ -4,6 +4,7 @@ GET/REQUEST Test with fifa example data
 filter
 --INI--
 filter.default=stripped
+error_reporting=E_ALL&~E_DEPRECATED
 --GET--
 id=f03_photos&pgurl=http%3A//fifaworldcup.yahoo.com/03/en/photozone/index.html
 --FILE--

ext/filter/tests/041.phpt

@@ -5,6 +5,7 @@ filter
 --INI--
 filter.default=stripped
 filter.default_flags=0
+error_reporting=E_ALL&~E_DEPRECATED
 --COOKIE--
 abc=dir; def=true; abc=root; xyz="foo bar";
 --FILE--

ext/filter/tests/filter_default_deprecation.phpt

@@ -0,0 +1,13 @@
+--TEST--
+filter.default is deprecated
+--INI--
+filter.default=special_chars
+--FILE--
+<?php
+
+echo "Done\n";
+
+?>
+--EXPECT--
+Deprecated: The filter.default ini setting is deprecated in Unknown on line 0
+Done