is_writeable函數bug問題

【題目】PHP的is_writeable()函數存在Bug，無法準確判斷一個目錄/檔案是否可寫，請寫一個函數來判斷目錄/檔案是否絕對可寫。
【等級】六級
【解決】 以下是CodeIgniter 中的is_really_writable函數解決方案，詳見函數註解
其中bug存在兩方面，
1.在windowns中，當檔案只有唯讀屬性時，is_writeable()函數才會傳回false，當傳回true時，該檔案不一定是可寫的。
如果是目錄，在目錄中新建檔案並透過開啟檔案來判斷；
如果是文件，可以透過開啟文件（fopen），來測試文件是否可寫入。
2.在Unix中，當php設定檔中開啟safe_mode時(safe_mode=on)，is_writeable()同樣不可用。
讀取設定檔是否safe_mode是否開啟。

<code><span>/**
 * Tests for file writability
 *
 * is_writable() returns TRUE on Windows servers when you really can't write to
 * the file, based on the read-only attribute.  is_writable() is also unreliable
 * on Unix servers if safe_mode is on.
 *
 *<span> @access</span>  private
 *<span> @return</span>  void
 */</span><span>if</span> ( ! function_exists(<span>'is_really_writable'</span>))
{
    <span><span>function</span><span>is_really_writable</span><span>(<span>$file</span>)</span>
    {</span><span>// If we're on a Unix server with safe_mode off we call is_writable</span><span>if</span> (DIRECTORY_SEPARATOR == <span>'/'</span><span>AND</span> @ini_get(<span>"safe_mode"</span>) == <span>FALSE</span>)
        {
            <span>return</span> is_writable(<span>$file</span>);
        }

        <span>// For windows servers and safe_mode "on" installations we'll actually</span><span>// write a file then read it.  Bah...</span><span>if</span> (is_dir(<span>$file</span>))
        {
            <span>$file</span> = rtrim(<span>$file</span>, <span>'/'</span>).<span>'/'</span>.md5(mt_rand(<span>1</span>,<span>100</span>).mt_rand(<span>1</span>,<span>100</span>));

            <span>if</span> ((<span>$fp</span> = @fopen(<span>$file</span>, FOPEN_WRITE_CREATE)) === <span>FALSE</span>)
            {
                <span>return</span><span>FALSE</span>;
            }

            fclose(<span>$fp</span>);
            @chmod(<span>$file</span>, DIR_WRITE_MODE);
            @unlink(<span>$file</span>);
            <span>return</span><span>TRUE</span>;
        }
        <span>elseif</span> ( ! is_file(<span>$file</span>) <span>OR</span> (<span>$fp</span> = @fopen(<span>$file</span>, FOPEN_WRITE_CREATE)) === <span>FALSE</span>)
        {
            <span>return</span><span>FALSE</span>;
        }

        fclose(<span>$fp</span>);
        <span>return</span><span>TRUE</span>;
    }
}</code>

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以上就介紹了is_writeable函數bug問題，包含了面向的內容，希望對PHP教學有興趣的朋友有幫助。