ext/standard: Fix memory leak in mail() when header key is numeric

Closes GH-20776

Author: Girgias

NEWS

@@ -57,6 +57,7 @@ PHP                                                                        NEWS
 
 - Standard:
   . Fix error check for proc_open() command. (ndossche)
+  . Fix memory leak in mail() when header key is numeric. (Girgias)
 
 18 Dec 2025, PHP 8.4.16
 

ext/standard/mail.c

@@ -214,7 +214,8 @@ PHPAPI zend_string *php_mail_build_headers(HashTable *headers)
 	ZEND_HASH_FOREACH_KEY_VAL(headers, idx, key, val) {
 		if (!key) {
 			zend_type_error("Header name cannot be numeric, " ZEND_LONG_FMT " given", idx);
-			break;
+			smart_str_free(&s);
+			return NULL;
 		}
 		ZVAL_DEREF(val);
 		/* https://tools.ietf.org/html/rfc2822#section-3.6 */

ext/standard/tests/mail/gh20776.phpt

@@ -0,0 +1,15 @@
+--TEST--
+GH-20776: mail() memory leak when header array contains numeric keys
+--FILE--
+<?php
+$to = "user@example.com";
+$subject = $message = "";
+
+try {
+    var_dump(mail($to, $subject, $message, ['RandomHeader' => 'Value', 5 => 'invalid key']));
+} catch (Throwable $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+?>
+--EXPECT--
+TypeError: Header name cannot be numeric, 5 given