xattr
PHP Manual

Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

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

xattr
PHP Manual