尽管具有777权限并且没有open_basedir值，但无法使用php写入/tmp

我正在尝试使用php fopen函数将文件写入我的/tmp目录(在apache服务器上)，但是失败:

I'm trying to write a file to my /tmp directory (on an apache server) with the php fopen function, but it fails:

<?php
$handle = fopen("/tmp/test.txt", "x");
if ($handle) 
   echo "Success!";
else 
    print_r(error_get_last());

这将返回错误消息:

无法打开流:没有这样的文件或目录.

failed to open stream: No such file or directory.

/tmp目录的权限设置为drwxrwxrwt，我可以看到Web用户正在向其写入其他文件.奇怪的是，如果我将脚本指向具有权限777的另一个文件夹，它将返回成功.但是我的open_basedir没有任何价值.我有safe_mode=on和allow_url_fopen=off，但我认为不应该解释吗?

The /tmp directory has permissions set to drwxrwxrwt and I can see that the web user is writing other files to it. Mysteriously, if I point the script to another folder with permissions 777, it returns success. But my open_basedir has no value. I have safe_mode=on and allow_url_fopen=off, but I don't think that should explain it?

这是Apache Httpd 2.0上的PHP 5.3.10.

This is PHP 5.3.10 on Apache Httpd 2.0.