Drupal 8 : how to write custom PHP functions and call the function from a custom page?

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PHP

Drupal

I am new to Drupal web development. I have already used Wordpress platform by building a custom plugin to write custom php functions, and calling the function from a custom page was easy.



How can I achieve similar with Drupal 8?
Do I need to create a custom module for writing the custom php functions?
How do I call a function from a custom page?

Please help.

What I have tried:

example:

<?php
function givenumber(){
return 50;
}
?>;

Which I call by using:

<?php  echo givenumber(); ?>