1-civicrm.settings-drupal-example.php

<?php

global $civicrm_paths;
$civicrm_paths['cms.root']['path']         = '/var/www/example.com';
$civicrm_paths['cms.root']['url']          = 'http://example.com';
$civicrm_paths['civicrm.root']['*']        = '[cms.root]/sites/all/modules/civicrm';
$civicrm_paths['civicrm.files']['*']       = '[cms.root]/sites/default/files/civicrm';
$civicrm_paths['civicrm.private']['*']     = '[cms.root]/sites/default/private/civicrm';
$civicrm_paths['civicrm.log']['*']         = '[civicrm.private]/configAndLogDir';
$civicrm_paths['civicrm.imageUpload']['*'] = '[civicrm.files]/persist/contribute';
$civicrm_paths['civicrm.phpCache']['*']    = '[civicrm.private]/templates_c';
$civicrm_paths['civicrm.assetCache']['*']  = '[civicrm.files]/dyn';
$civicrm_paths['civicrm.ckeditor']['*']    = '[civicrm.files]/ckeditor';
$civicrm_paths['civicrm.extension']['*']   = '[civicrm.files]/ext';
$civicrm_paths['civicrm.packages']['*']    = '[civicrm.root]/packages';

// Note: Each item in `$civicrm_paths` can specify any of these keys:
//  '*'     default formula 
//  'path'  local file path. (if omitted, use '*')
//  'url'   url (if omitted, use '*')

// Note: By default, everything is derived from `cms.root`.
// Another reasonable policy would be make everything concrete.
// It's really up to the installer.

// Note: Huzzah, in the average case, we don't have duplicate values
// like `imageUploadDir` and `imageUploadURL`.

1-civicrm.settings-wp-example.php

<?php

global $civicrm_paths;
$civicrm_paths['cms.root']['path']         = '/var/www/example.com';
$civicrm_paths['cms.root']['url']          = 'http://example.com';
$civicrm_paths['cms.admin']['url']         = '[cms.root]/wp-admin';
$civicrm_paths['civicrm.root']['*']        = '[cms.root]/wp-content/plugins/civicrm/civicrm';
$civicrm_paths['civicrm.files']['*']       = '[cms.root]/uploads/civicrm';
$civicrm_paths['civicrm.private']['*']     = '[cms.root]/uploads/_civicrm';
$civicrm_paths['civicrm.log']['*']         = '[civicrm.private]/configAndLogDir';
$civicrm_paths['civicrm.imageUpload']['*'] = '[civicrm.files]/persist/contribute';
$civicrm_paths['civicrm.phpCache']['*']    = '[civicrm.private]/templates_c';
$civicrm_paths['civicrm.assetCache']['*']  = '[civicrm.files]/dyn';
$civicrm_paths['civicrm.ckeditor']['*']    = '[civicrm.files]/ckeditor';
$civicrm_paths['civicrm.extension']['*']   = '[civicrm.files]/ext';
$civicrm_paths['civicrm.packages']['*']    = '[civicrm.root]/packages';

// Note: each item in `$civicrm_paths` can specify any of these keys:
//  '*'     default formula 
//  'path'  local file path. (if omitted, use '*')
//  'url'   url (if omitted, use '*')

// Note: by default, everything is derived from `cms.root`.
// Another reasonable policy would be make everything concrete.
// It's really up to the installer.

2-usage.php

<?php

Civi::paths()->getPath('[civicrm.log]');

Civi::paths()->getPath('[civicrm.imageUpload]/foo.jpg');
Civi::paths()->getUrl('[civicrm.imageUpload]/foo.jpg');

Civi::paths()->getPath('[civicrm.ckeditor]');
Civi::paths()->getUrl('[civicrm.ckeditor]');

3-CRM_Core_Config_MagicMerge.php

<?php

// To facilitate transition, CRM_Core_Config::singleton()->configAndLogDir should look
// in both `Civi::paths()` and the traditional location. For example, to __get('configAndLogDir'): 

if (Civi::paths()->hasVariable('[civicrm.log]')) {
  $value = Civi::paths()->getPath('[civicrm.log]');
} else {
  $value = oldLogic();
}