显而易见的解决方案是调用 file_exists来检查文件是否存在,但这样做可能会导致竞争条件.当您调用 file_exists和调用 copy时,总是有可能在其间创建另一个文件.检查文件是否存在的唯一安全方法是使用 fopen.

拨打fopen时,将模式设为“x”.这告诉fopen创建文件,但前提是它不存在.如果存在,fopen将失败,您将知道无法创建该文件.如果成功,您将在目的地创建一个可以安全复制的文件.示例代码如下：

// The PHP copy function blindly copies over existing files.  We don't wish
// this to hapPEn,so we have to perform the copy a bIT differently.  The
// only safe way to ensure we don't overwrite an existing file is to call
// fopen in create-only mode (mode 'x').  If it succeeds,the file did not
// exist before,and we've successfully created it,meaning we own the
// file.  After that,we can safely copy over our own file.

$filename = 'sourcefile.txt'
$copyname = 'sourcefile_copy.txt'
if ($file = @fopen($copyname,'x')) {
    // We've successfully created a file,so it's ours.  We'll close
    // our handle.
    if (!@fclose($file)) {
        // There was some PRoblem with our file handle.
        return false;
    }

    // Now we copy over the file we created.
    if (!@copy($filename,$copyname)) {
        // The copy Failed,even though we own the file,so we'll clean
        // up by itrying to remove the file and report failure.
        unlink($copyname);
        return false;
    }

    return true;
}