pts. How can I combine them?</span></p>
Use an inheritance chain. If you have two classes, say A in a.php:
<div class="doc-source">
<pre><code>require('fpdf.php');

class A extends FPDF
{
...
}</code></pre>
</div>
and B in b.php:
<div class="doc-source">
<pre><code>require('fpdf.php');

class B extends FPDF
{
...
}</code></pre>
</div>
then make B extend A:
<div class="doc-source">
<pre><code>require('a.php');

class B extends A
{
...
}</code></pre>
</div>
and make your own class extend B:
<div class="doc-so