PHP将$a的类型从integer更改为object.为什么？

<?php
$a = 3;
echo 'typeof $a is : ' . gettype($a) . "n"; // integer

$b = &$a;
echo 'typeof  $b es : ' . gettype($b) . "n"; // integer

$c = new stdClass;
$c->name = "charles";

$b = $c;
$b->name = "bill";

echo '$c->name : ' . $c->name . "n";
echo 'typeof $b es : ' . gettype($b) . "n";


echo 'typeof $a is : ' . gettype($a) . "n"; // object
echo 'The value of $a is : ' . $a->name; // bill
?>

输出：

typeof $a is : integer
typeof  $b is : integer
$c->name : bill
typeof  $b is : object
typeof $a is : object
The value of $a is : bill