PHP：如何在一些但不是所有页面上包含代码片段

I am new to PHP and programming in general and hope someone can help me with this.

I am working on building a website where the code for each page is saved in a separate file.

Now I have a couple of code snippets that I want to use on different pages but not on all of them. My current approach is that I paste them on every file that uses them which duplicates code and therefore supposedly is not good practice.

On the other hand when I use snippets on all pages I have removed them from the single files and store them as separate includes files using PHP's include or require in order to include them on a page, e.g. for the header, the footer and the menu etc. - example:

require_once("includes/header.php");

This works well and I was wondering if there is a similar way I can include the other code snippets as well BUT without having to save each of them as a separate file.

Is there a way for example that I could use functions for this or is there some other common practice ?

Example (just to show what I mean):

<?php
    // example of what I would like to include on different pages
    echo "<button type="button" class="class1" id="btn1">Button 1</button><br />
        <button type="button" class="class2" id="btn1">Button 2</button><br />
        <button type="button" class="class3" id="btn1">Button 3</button><br />";
?>

The pieces to insert could be anything but usually they are some small PHP / HTML snippets, like a set of buttons or a div or a dropdown etc.

Make the code snippets inside a function so you can call it on any other page by including the same page everytime. Let's say we have an index.php page, and we have a test.php that have this code (that we want to include on the index page):

    <?php
function hello(){
echo 'Hello World!';
}
hello(); //to print "hello world!" in this particular page
?>

Now, in the index.php page, we put:

<?php
include('test.php');
?>
<h1><?php hello(); ?></h1>

use a single index.php file to handle the rest

index.php

<?php
require_once 'header.php';
if(isset($_GET['page']){
    switch($_GET['page']){
        case "123":
         require_once 'snippet1.php';
        break;
        case "1234":
         require_once 'snippet2.php';
        break;
        default:
         require_once 'notfound.php';
    }
}
require_once 'footer.php';