如何使用PHP来显示HTML源代码

$html = "<div><h1>Hello World!</h1></div>";
$displayable_html = htmlspecialchars($html);
echo $displayable_html;

<div><h1>Hello World!</h1></div>

$html = "<div><h1>Hello World!</h1></div>";
highlight_string($html);

<code><span style="color: #000000">&lt;<span style="color: #0000BB">div</span>&gt;
&lt;<span style="color: #0000BB">h1</span>&gt;<span style="color: #DD0000">Hello World!</span>&lt;/<span style="color: #0000BB">h1</span>&gt;
&lt;/<span style="color: #0000BB">div</span>&gt;</span></code>