# 预定义常量

下列常量由此扩展定义，且仅在此扩展编译入 PHP 或在运行时动态载入时可用。

`XATTR_ROOT` (int)
  Set attribute in root (trusted) namespace. Requires root privileges.
`XATTR_DONTFOLLOW` (int)
  Do not follow the symbolic link but operate on symbolic link itself.
`XATTR_CREATE` (int)
  Function will fail if extended attribute already exists.
`XATTR_REPLACE` (int)
  Function will fail if extended attribute doesn't exist.

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