From 4811d4d608602112c852b421cfd1374efb6f7982 Mon Sep 17 00:00:00 2001
From: Weilin Du <weilindu@php.net>
Date: Sun, 15 Nov 2026 11:25:05 +0800
Subject: [PATCH] ext/intl: Fix Locale::lookup() ignoring the default locale
 for an empty language tag list

---
 NEWS                                          |  4 ++
 ext/intl/locale/locale_methods.c              |  3 ++
 .../locale_lookup_empty_language_tags.phpt    | 42 +++++++++++++++++++
 3 files changed, 49 insertions(+)
 create mode 100644 ext/intl/tests/locale_lookup_empty_language_tags.phpt

diff --git a/NEWS b/NEWS
index 64c61f8f121b..0b3edfc34628 100644
--- a/NEWS
+++ b/NEWS
@@ -52,6 +52,10 @@ PHP                                                                        NEWS
   . Fixed stack overflow in filter_var() and filter_var_array() with deeply
     nested arrays. (Eyüp Can Akman)
 
+- Intl:
+  . Fixed Locale::lookup() ignoring the default locale when the language tag
+    list is empty. (Weilin Du)
+
 - MBString:
   . Fixed stack overflow in mb_check_encoding() and mb_convert_encoding() with
     deeply nested arrays. (Eyüp Can Akman)
diff --git a/ext/intl/locale/locale_methods.c b/ext/intl/locale/locale_methods.c
index 2643f4920878..1b61794d67fb 100644
--- a/ext/intl/locale/locale_methods.c
+++ b/ext/intl/locale/locale_methods.c
@@ -1560,6 +1560,9 @@ PHP_FUNCTION(locale_lookup)
 	INTL_CHECK_LOCALE_LEN(loc_range_len);
 
 	if( !hash_arr || zend_hash_num_elements( hash_arr ) == 0 ) {
+		if (fallback_loc_str) {
+			RETURN_STR_COPY(fallback_loc_str);
+		}
 		RETURN_EMPTY_STRING();
 	}
 
diff --git a/ext/intl/tests/locale_lookup_empty_language_tags.phpt b/ext/intl/tests/locale_lookup_empty_language_tags.phpt
new file mode 100644
index 000000000000..b0bb2d1e6993
--- /dev/null
+++ b/ext/intl/tests/locale_lookup_empty_language_tags.phpt
@@ -0,0 +1,42 @@
+--TEST--
+Locale::lookup() honors the default locale for an empty language tag list
+--EXTENSIONS--
+intl
+--FILE--
+<?php
+foreach (['locale_lookup', [Locale::class, 'lookup']] as $lookup) {
+    foreach ([false, true] as $canonicalize) {
+        var_dump($lookup([], 'fr_FR', $canonicalize, 'en-US'));
+        var_dump($lookup([], '', $canonicalize, 'en-US'));
+        var_dump($lookup([], 'fr_FR', $canonicalize, ''));
+        var_dump($lookup([], 'fr_FR', $canonicalize, null));
+        var_dump($lookup([], 'fr_FR', $canonicalize));
+        var_dump(intl_get_error_code());
+    }
+}
+?>
+--EXPECT--
+string(5) "en-US"
+string(5) "en-US"
+string(0) ""
+string(0) ""
+string(0) ""
+int(0)
+string(5) "en-US"
+string(5) "en-US"
+string(0) ""
+string(0) ""
+string(0) ""
+int(0)
+string(5) "en-US"
+string(5) "en-US"
+string(0) ""
+string(0) ""
+string(0) ""
+int(0)
+string(5) "en-US"
+string(5) "en-US"
+string(0) ""
+string(0) ""
+string(0) ""
+int(0)