Using https://github.com/php-test-helpers/php-test-helpers we can rename functions to help unit testing. This is a utility function.

stub.php

<?php

function stub_function($function) {
  if (!function_exists($function)) {
    // Create dummy function.
    $string = 'function ' . $function . "() {}";
    eval($string);
  }

  rename_function($function, $function . '_orig');
  rename_function($function . '_stub', $function);
}