<?php

$str = "Loreum Ipsum Dolor Sit Amet <code>Some HTML Code like <h1>Head</h1></code>. Loreum Ipsum <code>Some Code</code> Dolor Sit Amet.";
$str = preg_replace_callback(
    '|(?s)(?<=<code>).*?(?=</code>)|',
    function ($matches) {
        return str_replace('<', '&lt;', $matches[0]);
    },
    $str
);
echo $str;