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.