diff --git a/ext/mbstring/mbstring.c b/ext/mbstring/mbstring.c
index 12ec4411768c1..8f65681c3cee1 100644
--- a/ext/mbstring/mbstring.c
+++ b/ext/mbstring/mbstring.c
@@ -3129,7 +3129,7 @@ static void php_do_mb_trim(INTERNAL_FUNCTION_PARAMETERS, mb_trim_mode mode)
 	ZEND_PARSE_PARAMETERS_START(1, 3)
 		Z_PARAM_STR(str)
 		Z_PARAM_OPTIONAL
-		Z_PARAM_STR(what)
+		Z_PARAM_STR_OR_NULL(what)
 		Z_PARAM_STR_OR_NULL(encoding)
 	ZEND_PARSE_PARAMETERS_END();
 
diff --git a/ext/mbstring/mbstring.stub.php b/ext/mbstring/mbstring.stub.php
index 6eb041bf1eba2..af9c5cbb93ea2 100644
--- a/ext/mbstring/mbstring.stub.php
+++ b/ext/mbstring/mbstring.stub.php
@@ -139,11 +139,11 @@ function mb_ucfirst(string $string, ?string $encoding = null): string {}
 
 function mb_lcfirst(string $string, ?string $encoding = null): string {}
 
-function mb_trim(string $string, string $characters = " \f\n\r\t\v\x00\u{00A0}\u{1680}\u{2000}\u{2001}\u{2002}\u{2003}\u{2004}\u{2005}\u{2006}\u{2007}\u{2008}\u{2009}\u{200A}\u{2028}\u{2029}\u{202F}\u{205F}\u{3000}\u{0085}\u{180E}", ?string $encoding = null): string {}
+function mb_trim(string $string, ?string $characters = null, ?string $encoding = null): string {}
 
-function mb_ltrim(string $string, string $characters = " \f\n\r\t\v\x00\u{00A0}\u{1680}\u{2000}\u{2001}\u{2002}\u{2003}\u{2004}\u{2005}\u{2006}\u{2007}\u{2008}\u{2009}\u{200A}\u{2028}\u{2029}\u{202F}\u{205F}\u{3000}\u{0085}\u{180E}", ?string $encoding = null): string {}
+function mb_ltrim(string $string, ?string $characters = null, ?string $encoding = null): string {}
 
-function mb_rtrim(string $string, string $characters = " \f\n\r\t\v\x00\u{00A0}\u{1680}\u{2000}\u{2001}\u{2002}\u{2003}\u{2004}\u{2005}\u{2006}\u{2007}\u{2008}\u{2009}\u{200A}\u{2028}\u{2029}\u{202F}\u{205F}\u{3000}\u{0085}\u{180E}", ?string $encoding = null): string {}
+function mb_rtrim(string $string, ?string $characters = null, ?string $encoding = null): string {}
 
 /** @refcount 1 */
 function mb_detect_encoding(string $string, array|string|null $encodings = null, bool $strict = false): string|false {}
diff --git a/ext/mbstring/mbstring_arginfo.h b/ext/mbstring/mbstring_arginfo.h
index f40715b13ab7c..a4c63aa2824e1 100644
--- a/ext/mbstring/mbstring_arginfo.h
+++ b/ext/mbstring/mbstring_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit the .stub.php file instead.
- * Stub hash: ea642b9010bc38a3b13710662fef48663d4385e1 */
+ * Stub hash: 03c07f68bea7d7b96e6dc11f180f45663b859ed3 */
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_MASK_EX(arginfo_mb_language, 0, 0, MAY_BE_STRING|MAY_BE_BOOL)
 	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, language, IS_STRING, 1, "null")
@@ -124,7 +124,7 @@ ZEND_END_ARG_INFO()
 
 ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX(arginfo_mb_trim, 0, 1, IS_STRING, 0)
 	ZEND_ARG_TYPE_INFO(0, string, IS_STRING, 0)
-	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, characters, IS_STRING, 0, "\" \\f\\n\\r\\t\\v\\x00             

  　…᠎\"")
+	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, characters, IS_STRING, 1, "null")
 	ZEND_ARG_TYPE_INFO_WITH_DEFAULT_VALUE(0, encoding, IS_STRING, 1, "null")
 ZEND_END_ARG_INFO()
 
diff --git a/ext/mbstring/tests/gh13815.phpt b/ext/mbstring/tests/gh13815.phpt
new file mode 100644
index 0000000000000..f5d49c0aa1d28
--- /dev/null
+++ b/ext/mbstring/tests/gh13815.phpt
@@ -0,0 +1,20 @@
+--TEST--
+GH-13815 (mb_trim() inaccurate $characters default value)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+$strUtf8 = "\u{3042}\u{3000}"; // U+3000: fullwidth space
+var_dump(mb_strlen(mb_trim($strUtf8)));
+var_dump(mb_strlen(mb_trim($strUtf8, encoding: 'UTF-8')));
+
+mb_internal_encoding('Shift_JIS');
+$strSjis = mb_convert_encoding($strUtf8, 'Shift_JIS', 'UTF-8');
+var_dump(mb_strlen(mb_trim($strSjis)));
+var_dump(mb_strlen(mb_trim($strSjis, encoding: 'Shift_JIS')));
+?>
+--EXPECT--
+int(1)
+int(1)
+int(1)
+int(1)