Sometimes you may need to “filter” the value of the_title(). This can be done with the_title filter. Below are a few examples on how it can be used:

The WordPress Way

<?php
function custom_title_filter($title) {
  // prepend "foo" to title string
  $title = "foo" . $title;

  // return updated title string
  return $title;
}
add_filter('the_title', 'custom_title_filter');

The OOP Way

<?php
class Custom_Filters {

  function __construct() {
    add_filter('the_title', [$this, 'custom_title_filter']);
  }

  function custom_title_filter($title) {
    // prepend "foo" to title string
    $title = "foo" . $title;

    // return updated title string
    return $title;
  }
}
new Custom_Filters();

The Anonymous Way

<?php
add_filter('the_title', function($title) {
  // prepend "foo" to title string
  $title = "foo" . $title;

  // return updated title string
  return $title;
});

All three of the above options do the same thing, there’ just implemented with different styles. If you are not sure which style to go with I recommend using option one: The WordPress Way. When you get into plugin development you will find many developers opt for the OOP Way.