diff --git a/NEWS b/NEWS
index 3c4d6993f46c..5fc78a655157 100644
--- a/NEWS
+++ b/NEWS
@@ -73,6 +73,8 @@ PHP                                                                        NEWS
     types. (Weilin Du)
   . Fixed MessageFormatter::parse() and parseMessage() returning PHP_INT_MIN
     as float rather than int on 64-bit platforms. (Weilin Du)
+  . Fixed UConverter::transcode() silently truncating from_subst and to_subst
+    option lengths greater than 127 bytes. (Weilin Du)
 
 - JSON:
   . Enriched JSON last error / exception message with error location.
diff --git a/UPGRADING b/UPGRADING
index 95299dd5117f..543e1996e1b0 100644
--- a/UPGRADING
+++ b/UPGRADING
@@ -44,6 +44,9 @@ PHP 8.6 UPGRADE NOTES
     int, rather than float, on 64-bit platforms when parsing integer values.
   . The $type parameter of IntlBreakIterator::getPartsIterator() has been
     changed from string to int to match the underlying implementation.
+  . UConverter::transcode() now rejects from_subst and to_subst option values
+    longer than 127 bytes instead of silently truncating the length before
+    passing it to ICU.
 
 - PCNTL:
   . pcntl_alarm() now raises a ValueError if the seconds argument is
diff --git a/ext/intl/converter/converter.cpp b/ext/intl/converter/converter.cpp
index 597746b8faca..88f05136bbbd 100644
--- a/ext/intl/converter/converter.cpp
+++ b/ext/intl/converter/converter.cpp
@@ -682,6 +682,16 @@ static zend_string* php_converter_do_convert(UConverter *dest_cnv,
 }
 /* }}} */
 
+static void php_converter_set_subst_chars(UConverter *cnv, zend_string *subst, UErrorCode *error)
+{
+	if (ZSTR_LEN(subst) > SCHAR_MAX) {
+		*error = U_ILLEGAL_ARGUMENT_ERROR;
+		return;
+	}
+
+	ucnv_setSubstChars(cnv, ZSTR_VAL(subst), (int8_t) ZSTR_LEN(subst), error);
+}
+
 /* {{{ */
 #define UCNV_REASON_CASE(v) case (UCNV_ ## v) : RETURN_STRINGL( "REASON_" #v , sizeof( "REASON_" #v ) - 1);
 PHP_METHOD(UConverter, reasonText) {
@@ -761,13 +771,13 @@ PHP_METHOD(UConverter, transcode) {
 				(tmpzval = zend_hash_str_find_deref(Z_ARRVAL_P(options), "from_subst", sizeof("from_subst") - 1)) != NULL &&
 				Z_TYPE_P(tmpzval) == IS_STRING) {
 				error = U_ZERO_ERROR;
-				ucnv_setSubstChars(src_cnv, Z_STRVAL_P(tmpzval), Z_STRLEN_P(tmpzval) & 0x7F, &error);
+				php_converter_set_subst_chars(src_cnv, Z_STR_P(tmpzval), &error);
 			}
 			if (U_SUCCESS(error) &&
 				(tmpzval = zend_hash_str_find_deref(Z_ARRVAL_P(options), "to_subst", sizeof("to_subst") - 1)) != NULL &&
 				Z_TYPE_P(tmpzval) == IS_STRING) {
 				error = U_ZERO_ERROR;
-				ucnv_setSubstChars(dest_cnv, Z_STRVAL_P(tmpzval), Z_STRLEN_P(tmpzval) & 0x7F, &error);
+				php_converter_set_subst_chars(dest_cnv, Z_STR_P(tmpzval), &error);
 			}
 		}
 
diff --git a/ext/intl/rangeformatter/rangeformatter_class.cpp b/ext/intl/rangeformatter/rangeformatter_class.cpp
index 6921791e588f..2df252d1986b 100644
--- a/ext/intl/rangeformatter/rangeformatter_class.cpp
+++ b/ext/intl/rangeformatter/rangeformatter_class.cpp
@@ -60,6 +60,19 @@ zend_object *IntlNumberRangeFormatter_object_create(zend_class_entry *ce)
     return &intern->zo;
 }
 
+static icu::Formattable rangeformatter_create_formattable(zval *number)
+{
+    icu::Formattable formattable;
+
+    if (Z_TYPE_P(number) == IS_DOUBLE) {
+        formattable.setDouble(Z_DVAL_P(number));
+    } else {
+        formattable.setInt64(static_cast<int64_t>(Z_LVAL_P(number)));
+    }
+
+    return formattable;
+}
+
 U_CFUNC PHP_METHOD(IntlNumberRangeFormatter, __construct)
 {
     ZEND_PARSE_PARAMETERS_NONE();
@@ -154,8 +167,8 @@ U_CFUNC PHP_METHOD(IntlNumberRangeFormatter, format)
 
     UErrorCode error = U_ZERO_ERROR;
 
-    icu::Formattable start_formattable(Z_TYPE_P(start) == IS_DOUBLE ? Z_DVAL_P(start) : Z_LVAL_P(start));
-    icu::Formattable end_formattable(Z_TYPE_P(end) == IS_DOUBLE ? Z_DVAL_P(end) : Z_LVAL_P(end));
+    icu::Formattable start_formattable = rangeformatter_create_formattable(start);
+    icu::Formattable end_formattable = rangeformatter_create_formattable(end);
 
     UnicodeString result = RANGEFORMATTER_OBJECT(obj)->formatFormattableRange(start_formattable, end_formattable, error).toString(error);
 
diff --git a/ext/intl/tests/rangeformatter/rangeformatter_format_int64.phpt b/ext/intl/tests/rangeformatter/rangeformatter_format_int64.phpt
new file mode 100644
index 000000000000..9cefbcfb1ea8
--- /dev/null
+++ b/ext/intl/tests/rangeformatter/rangeformatter_format_int64.phpt
@@ -0,0 +1,27 @@
+--TEST--
+IntlNumberRangeFormatter::format() preserves int64 precision
+--EXTENSIONS--
+intl
+--SKIPIF--
+<?php
+if (!class_exists('IntlNumberRangeFormatter')) {
+    die('skip IntlNumberRangeFormatter not available');
+}
+if (PHP_INT_SIZE < 8) {
+    die('skip 64-bit only');
+}
+?>
+--FILE--
+<?php
+$formatter = IntlNumberRangeFormatter::createFromSkeleton(
+    '',
+    'en_US',
+    IntlNumberRangeFormatter::COLLAPSE_AUTO,
+    IntlNumberRangeFormatter::IDENTITY_FALLBACK_SINGLE_VALUE
+);
+
+$formatted = $formatter->format(9007199254740993, 9007199254740993);
+var_dump(preg_replace('/[^0-9]/', '', $formatted));
+?>
+--EXPECT--
+string(16) "9007199254740993"
diff --git a/ext/intl/tests/uconverter_transcode_subst_length.phpt b/ext/intl/tests/uconverter_transcode_subst_length.phpt
new file mode 100644
index 000000000000..93001b16680c
--- /dev/null
+++ b/ext/intl/tests/uconverter_transcode_subst_length.phpt
@@ -0,0 +1,20 @@
+--TEST--
+UConverter::transcode() rejects too long substitution strings
+--EXTENSIONS--
+intl
+--INI--
+intl.use_exceptions=false
+--FILE--
+<?php
+$subst = str_repeat('A', 129);
+
+var_dump(UConverter::transcode('abc', 'UTF-8', 'ASCII', ['from_subst' => $subst]));
+echo intl_get_error_message(), "\n";
+var_dump(UConverter::transcode('abc', 'UTF-8', 'ASCII', ['to_subst' => $subst]));
+echo intl_get_error_message(), "\n";
+?>
+--EXPECT--
+bool(false)
+UConverter::transcode(): returned error 1: U_ILLEGAL_ARGUMENT_ERROR: U_ILLEGAL_ARGUMENT_ERROR
+bool(false)
+UConverter::transcode(): returned error 1: U_ILLEGAL_ARGUMENT_ERROR: U_ILLEGAL_ARGUMENT_ERROR