序列化对象输出奇怪的符号

<?php
class test {
  private $a;
}

$test = new test;
$test = serialize($test);
echo "<a href=$test>$test</a>";

A string from the resulting markup:

<a href="O:4:&quot;test&quot;:1:{s:7:&quot;�test�a&quot;;N;}">O:4:"test":1:{s:7:"testa";N;}</a>

Question: why the value of the same variable output as a value of href attribute is seen WITH these � symbols while placed as content of the anchor tag is seen WITHOUT these � symbols?