Fix #79467: data:// wrappers are writable

Despite the docs claiming that data: wrappers would not be writable[1],
they are implemented as writing to a memory stream.  That does not seem
to be particularly sensible, so we disallow writing altogether.

[1] <https://www.php.net/manual/en/wrappers.data.php#refsect1-wrappers.data-options>

Author: cmb69

NEWS

@@ -24,6 +24,7 @@ PHP                                                                        NEWS
     (cmb)
   . Use SSE2 instructions do locale independent strtolower. (Laruence)
   . Fixed bug #79521 (Check __set_state structure). (carusogabriel)
+  . Fixed bug #79467 (data:// wrappers are writable). (cmb)
 
 - BZ2:
   . Fixed bug #71263 (fread() does not report bzip2.decompress errors). (cmb)

UPGRADING

@@ -181,6 +181,7 @@ PHP 8.0 UPGRADE NOTES
   . Disabled functions are now treated exactly like non-existent functions.
     Calling a disabled function will report it as unknown, and redefining a
     disabled function is now possible.
+  . data: wrappers are no longer writable, what matches the documented behavior.
 
 - COM:
   . Removed the ability to import case-insensitive constants from type

ext/standard/tests/streams/bug79467.phpt

@@ -0,0 +1,9 @@
+--TEST--
+Bug #79467 (data:// wrappers are writable)
+--FILE--
+<?php
+var_dump(file_put_contents('data://text/plain,cccc', 'data'));
+?>
+--EXPECTF--
+Notice: file_put_contents(): Stream is not writable in %s on line %d
+bool(false)

main/streams/memory.c

@@ -619,7 +619,7 @@ PHPAPI php_stream *_php_stream_temp_open(int mode, size_t max_memory_usage, char
 /* }}} */
 
 PHPAPI const php_stream_ops php_stream_rfc2397_ops = {
-	php_stream_temp_write, php_stream_temp_read,
+	NULL, php_stream_temp_read,
 	php_stream_temp_close, php_stream_temp_flush,
 	"RFC2397",
 	php_stream_temp_seek,