Fix #68471: IntlDateFormatter fails for "GMT+00:00" timezone

GMT+00:00 is recognized by ICU, and is normalized to GMT.  There are no
issues when GMT+00:00 is passed to `IntlTimeZone::createTimeZone()`,
but passing it to IntlDateFormatter::__construct() causes a failure,
since there is an additional check regarding the validity.  While
checking the validity of the result of `TimeZone::createTimeZone()`[1]
is a good idea, comparing the IDs is overly restrictive.  Instead we
just check that the timezone is supported by ICU.

[1] <https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/classicu_1_1TimeZone.html#a35da0507b62754ffe5d8d59c19775cdb>

Closes GH-7190.

Author: cmb69

NEWS

@@ -11,6 +11,7 @@ PHP                                                                        NEWS
 - Intl:
   . Fixed bug #72809 (Locale::lookup() wrong result with canonicalize option).
     (cmb)
+  . Fixed bug #68471 (IntlDateFormatter fails for "GMT+00:00" timezone). (cmb)
 
 - PCRE:
   . Fixed bug #81101 (PCRE2 10.37 shows unexpected result). (Anatol)

ext/intl/tests/bug68471.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Bug #68471 (IntlDateFormatter fails for "GMT+00:00" timezone)
+--SKIPIF--
+<?php
+if (!extension_loaded('intl')) die("sikp intl extension not available");
+?>
+--FILE--
+<?php
+$formatter = new IntlDateFormatter(
+    'fr_FR',
+    IntlDateFormatter::NONE,
+    IntlDateFormatter::NONE,
+    "GMT+00:00"
+);
+var_dump($formatter);
+?>
+--EXPECT--
+object(IntlDateFormatter)#1 (0) {
+}

ext/intl/timezone/timezone_class.cpp

@@ -176,8 +176,7 @@ U_CFUNC TimeZone *timezone_process_timezone_argument(zval *zv_timezone,
 		return timezone_convert_datetimezone(tzobj->type, tzobj, 0,
 			outside_error, func);
 	} else {
-		UnicodeString	id,
-						gottenId;
+		UnicodeString	id;
 		UErrorCode		status = U_ZERO_ERROR; /* outside_error may be NULL */
 		if (!try_convert_to_string(zv_timezone)) {
 			zval_ptr_dtor_str(&local_zv_tz);
@@ -204,7 +203,7 @@ U_CFUNC TimeZone *timezone_process_timezone_argument(zval *zv_timezone,
 			zval_ptr_dtor_str(&local_zv_tz);
 			return NULL;
 		}
-		if (timeZone->getID(gottenId) != id) {
+		if (*timeZone == TimeZone::getUnknown()) {
 			spprintf(&message, 0, "%s: no such time zone: '%s'",
 				func, Z_STRVAL_P(zv_timezone));
 			if (message) {