Why do the class lists returned by spl_classes() and get_declared_classes() differ? How to understand the differences?

<span class="hljs-meta"><?php
// The start of the article, not related to the main content
]]]
<article><pre class="overflow-visible!"><code class="codes"><span><span class="hljs-meta"><?php
// The introductory part of the article, where you can add some quotes, explanations, or background
?>
<hr>
<p>## Why do the class lists returned by spl_classes() and get_declared_classes() differ? How to understand the differences?<span></p>
<p>In PHP, we often need to view the currently declared classes for debugging or dynamic manipulation. Both <code>spl_classes()