Fix #79908: json_encode encodes negative zero as int

Encoding a negative zero as `-0` is likely to loose the sign when
decoding (at least it does with `json_decode()`).  Therefore, we encode
it as if `JSON_PRESERVE_ZERO_FRACTION` was specified, i.e. as `-0.0`.

Closes phpGH-7234.

Author: cmb69

NEWS

@@ -21,6 +21,9 @@ PHP                                                                        NEWS
   . Fixed bug #68471 (IntlDateFormatter fails for "GMT+00:00" timezone). (cmb)
   . Fixed bug #74264 (grapheme_strrpos() broken for negative offsets). (cmb)
 
+- JSON:
+  . Fixed bug #79908 (json_encode encodes negative zero as int). (cmb)
+
 - OpenSSL:
   . Fixed bug #52093 (openssl_csr_sign truncates $serial). (cmb)
 

ext/json/json_encoder.c

@@ -104,7 +104,8 @@ static inline void php_json_encode_double(smart_str *buf, double d, int options)
 
 	php_gcvt(d, (int)PG(serialize_precision), '.', 'e', num);
 	len = strlen(num);
-	if (options & PHP_JSON_PRESERVE_ZERO_FRACTION && strchr(num, '.') == NULL && len < PHP_DOUBLE_MAX_LENGTH - 2) {
+	if ((options & PHP_JSON_PRESERVE_ZERO_FRACTION && strchr(num, '.') == NULL && len < PHP_DOUBLE_MAX_LENGTH - 2)
+	 || (UNEXPECTED(len == 2 && num[0] == '-' && num[1] == '0'))) {
 		num[len++] = '.';
 		num[len++] = '0';
 		num[len] = '\0';

ext/json/tests/bug79908.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Bug #79908 (json_encode encodes negative zero as int)
+--SKIPIF--
+<?php
+if (!extension_loaded('json')) die("skip json extension not available");
+?>
+--FILE--
+<?php
+var_dump(json_encode(-0.));
+?>
+--EXPECT--
+string(4) "-0.0"