问个基础问题，关于上传文件
s_uploaded_file和 file_exists都是检查文件存在不？参数都是路径，那为什么上传文件一定要用is_uploaded_file呢？
回复讨论(解决方案)
呵呵，这是一个有历史渊源的问题。使用 is_uploaded_file 也完全是出于自我保护
php 提供了两个处理上传文件的方案
move_uploaded_file
和
is_uploaded_file
copy 或 rename
copy 和 rename 都是文件系统函数，当然不只是针对上传文件的
所以你完全可以用 copy 函数将 linux 的用户口令文件 etc/password 做一个副本
php 还提供了一个 register_globals 开关，用于将传入的变量自动导入全局变量表（php5.4开始不可用了）
当此开关打开时
将产生变量 $file_name、$file_tmp、$file_type....等变量
而upload.php?file_tmp=etc/password 也将产生 $file_tmp 变量
那么你就需要准确的知道这个 $file_tmp 是由 type=file 产生的还是由 URL参数产生的了
当然，这种情况并不会发生，因为 php 还有一重保护机制（这个留给你自己发现吧）
但是由于代码是千变万化的，保守一点比较稳妥。指不定哪就出现了危机
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