预定义常量
PHP Manual

posix_access() constants

Note:

These constants are available starting with PHP 5.1.0. Please note that some of them may not be available on your system.

POSIX_F_OK (integer)
Check whether the file exists.
POSIX_R_OK (integer)
Check whether the file exists and has read permissions.
POSIX_W_OK (integer)
Check whether the file exists and has write permissions.
POSIX_X_OK (integer)
Check whether the file exists and has execute permissions.

预定义常量
PHP Manual