Token replacement in newsletter body using php to retrieve subscriber email for view insertion

You can't use tokes in PHP code. Tokens are only replaced *after* the template is rendered, not before.

Quoting the documentation of that template:

 * - $simplenews_subscriber: The subscriber for which the newsletter is built.
 *   Note that depending on the used caching strategy, the generated body might
 *   be used for multiple subscribers. If you created personalized newsletters
 *   and can't use tokens for that, make sure to disable caching or write a
 *   custom caching strategy implemention.

So, using $simplenews_subscriber->mail (or better, ->snid, because you have that too) should work but make sure to read the notes about caching.

I confirm the simplenews token replacement occurs only after the filtering process.

Thus, the [simplenews-subscriber:...] token cannot be evaluated in php as you tried to do, and even if you use the Token Filter, it will not be processed, because when text filtering occurs, the token contextual object 'simplenews-subscriber' is unknown: this token will be replaced later, by this code located in simplenews/includes/simplenews.source.inc, class SimplenewsSourceNode, method getBodyWithFormat() (line 728 in 7.x-1.0):

protected function getBodyWithFormat($format) {
  ...
  $body = token_replace($body, $this->getTokenContext(), array('sanitize' => FALSE));

Here the 'simplenews-subscriber' object is provided as second argument for token_replace(), I didn't find a way to retrieve it elsewhere.

Therefore, I think the only way to insert values depending on the recipient (email, or - my need - a view with the recipient uid as a parameter), is to use the token mechanics and provide a 'simplenews-subscriber' or better, a 'simplenews-subscriber:user' token (because the value provided by the token relates to a user).

For instance, I created a custom module providing the result of a view through a custom token [simplenews-subscriber:user:MYMODULE-newsletter-registrations].

/**
 * Implements hook_token_info()
 */
function MYMODULE_token_info() {
  $types = array();  // no new token type
  // tokens for inserting parametrized views
  $tokens = array();
  $tokens['MYMODULE-newsletter-registrations'] = array(
    'name' => t('Insert view: Registrations'),
    'description' => t('Insert view: List of the registrations for the newsletter recipient.'),
  );
  return array(
    'types' => $types,
    'tokens' => array(
      'user' => $tokens,
    ),
  );
}

/**
 * Implements hook_tokens()
 */
function MYMODULE_tokens($type, $tokens, array $data = array(), array $options = array()) {
  $replacements = array();
  switch ($type) {
    case 'user':
      if (!isset($data['user'])) {
        return;
      }
      $subscriber = $data['user'];
      foreach ($tokens as $name => $original) {
        switch ($name) {
          case 'MYMODULE-newsletter-registrations':
            $replacements[$original] = MYMODULE__render_view('conferences','user_subscriptions',array($subscriber->uid));
            break;
        }
      }
  }
  return $replacements;
}

function MYMODULE__render_view($view_id,$display_id,$args=NULL,$hide_contextual_links=TRUE) {
  $view = views_get_view($view_id);
  if (!$view || !$view->access($display_id)) {
    return;
  }
  $view->hide_admin_links = $hide_contextual_links;
  return $view->preview($display_id, $args);
}

---
Note for users trying to insert a parametrized view with Insert View: it's also impossible, for the same reasons, the [simplenews-subscriber:...] token being unavailable during filtering. It's the reason why I went to the above solution.
See also issue #302612 'Dynamic Arguments?' of Insert Views.

I just discovered the Views Send module, which is very much appropriate to send content dynamically computed for the recipient user...
It's a better solution for my needs than simplenews and my token hooks.