How does php determine whether a constant is defined?

Use the defined function

defined(String constants_name)

If the constant exists, return bool(true)

If it does not exist, return bool(false)

<?php 
define("name",unique);
$p = "name";
$is1 = defined($p);
$is2 = defined("p2");

var_dump($is1);
var_dump($is2);
?>

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