General
-------

Visitor Pattern
http://butunclebob.com/ArticleS.UncleBob.IuseVisitor


WordPress MU
------------
http://codex.wordpress.org/Create_A_Network
http://stackoverflow.com/questions/13960514/how-to-adapt-my-plugin-to-multisite/


Wordpress
---------

Internationalizing a Plugin
http://codex.wordpress.org/I18n_for_WordPress_Developers

Internationalization: You're probably doing it wrong
http://ottopress.com/2012/internationalization-youre-probably-doing-it-wrong/

Portable WordPress-only alternative to curl_init
http://planetozh.com/blog/2009/08/how-to-make-http-requests-with-wordpress/

"Once the action is complete. I want the page to redirect back to the form that called the script."
http://stackoverflow.com/questions/21450966/wp-redirect-call-to-undefined-function-new-plugin

"Data Sanitization and Validation With WordPress"
http://code.tutsplus.com/articles/data-sanitization-and-validation-with-wordpress--wp-25536

Tutorials
---------

"SMTP with oauth for gmail." - Aug 2012
https://skroesbergen.wordpress.com/2012/08/22/smtp-with-oauth-for-gmail/

"GOOGLE, SMTP, AND OAUTH2 AUTHENTICATION" - Feb 2013
http://www.chargenevier.com/2013/02/google-smtp-and-oauth2-authentication/

PHP, SMTP, XOAUTH2 and Gmail - Feb 2014
http://www.boxuk.com/blog/php-smtp-xoauth2-gmail/

SASL XOAUTH2 protocol documentation
https://developers.google.com/gmail/xoauth2_protocol


Creating a Gmail Token
----------------------

Google Developer OAuth 2.0 Playground
https://developers.google.com/oauthplayground/

Google Developers Console
https://console.developers.google.com/

Google Apps Security Settings
https://admin.google.com/AdminHome?chromeless=1&fral=1#SecuritySettings:

Account Permissions
https://security.google.com/settings/security/permissions?pli=1

Rereshing a Token
-----------------
How do I refresh my google_oauth2 access token using my refresh token?
http://stackoverflow.com/questions/12792326/how-do-i-refresh-my-google-oauth2-access-token-using-my-refresh-token



References
----------

Using OAuth 2.0 to Access Google APIs
https://developers.google.com/accounts/docs/OAuth2

Gmail Platform - OAuth 2.0
https://developers.google.com/gmail/oauth_protocol?hl=nl


OAuth Overview
--------------

Request Permission ->
<- Authorization Grant

Grant Code ->
<- Authorization Token

(Access Token and Refresh Token)


wp_mail() and Email in General
-----------------------------

The PLAIN Simple Authentication and Security Layer (SASL) Mechanism
https://www.ietf.org/rfc/rfc4616.txt

What is the difference between ports 465 and 587?
http://stackoverflow.com/a/19942206/4368109

The Return Path
https://wordpress.org/ideas/topic/wp-mail-and-php-mail-function-according-to-rfc-822

Hacking WordPress Emails
http://glennmessersmith.com/pages/wpemail.html

The Importance of Content-Type Character Encoding in HTML Emails
http://www.emailonacid.com/blog/details/C13/the_importance_of_content-type_character_encoding_in_html_emails

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)
http://www.joelonsoftware.com/articles/Unicode.html

How to Tell Who Supports SMTP TLS for Email Transmission
https://luxsci.com/blog/how-to-tell-who-supports-tls-for-email-transmission.html

JavaMail: Keeping IMAPFolder.idle() alive
http://stackoverflow.com/questions/4155412/javamail-keeping-imapfolder-idle-alive

Reach More People and Improve Your Spam Score: Why Multi-Part Email is Important
https://litmus.com/blog/reach-more-people-and-improve-your-spam-score-why-multi-part-email-is-important

HTTP Message Headers
http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

Sending a multipart E-Mail using PHP Mailer/wp_mail - not all clients display E-Mail
http://stackoverflow.com/questions/22614615/sending-a-multipart-e-mail-using-php-mailer-wp-mail-not-all-clients-display-e

The AUTH Command
http://www.samlogic.net/articles/smtp-commands-reference-auth.htm

SMTP Authentication with Zend
http://framework.zend.com/manual/1.12/en/zend.mail.smtp-authentication.html

SMTP Authentication [Tutorial]
http://www.fehcom.de/qmail/smtpauth.html



Zend Mail
---------

Getting access to the SMTP session transcript
http://zend-framework-community.634137.n4.nabble.com/Zend-Mail-and-SMTP-Status-Code-td644679.html

Sending Mail as a background process
http://stackoverflow.com/questions/14177500/sending-emails-in-php-and-using-google-apps-mail-as-smtp-is-too-slow



PHP Resources
-------------
Why your PHP App NEEDS a Circuit Breaker
http://devzone.zend.com/1424/why-your-php-app-needs-a-circuit-breaker/

Capturing and Handling PHP Errors Gracefully
http://biostall.com/capturing-and-handling-php-errors-gracefully

Gmail SMTP Settings and New Security Measures Demystified
http://www.wpsitecare.com/gmail-smtp-settings/



Google API
----------

Gmail API
https://developers.google.com/gmail/api/

Google APIs Client Library for PHP
https://github.com/google/google-api-php-client

(Google App Engine) Mail PHP API Overview
https://cloud.google.com/appengine/docs/php/mail/

(Google App Engine) Bulk Mail with Analytics via SendGrid
https://cloud.google.com/appengine/docs/php/mail/sendgrid
