<?php

error_reporting(-1);

define('FOO', 1);
var_dump(FOO);
define('FOO', 2);
var_dump(FOO);

define('BAR', 1, true);
var_dump(BAR);
define('BAR', 2);
var_dump(BAR);

define('baz', 1, true);
var_dump(baz);
define('baz', 2);
var_dump(baz);

int(1)

Notice: Constant FOO already defined in /t.php on line 7
int(1)
int(1)
int(2)
int(1)

Notice: Constant baz already defined in /t.php on line 17
int(1)