Wordpress, Joomla and Drupal are not good examples of "good" PHP code. They work, and are used extensively, but experienced developers who care about code quality blow a gasket when they look at the source code.

To do proper PHP development you should have a local VM. I created something to help in that regard[0]. It's been fairly well received.

There's also websites like PHP The Right Way [1] that act like a collection of best practices. If you're not using Composer[2] you should start doing so. I wish Javascript had a package manager as robust and useful as Composer (I know of Bower, but imo it's not nearly as good!). PHP Developer[3] is a source of good daily blog posts. The ones I read on a regular basis are ircmaxell [4], fabpot[5], lornajane[6], philsturgeon[7], richardmiller[8], and phparch[9]. I also sometimes write some things[10].

[0] https://puphpet.com

[1] http://phptherightway.com

[2] http://getcomposer.org

[3] http://phpdeveloper.org

[4] http://blog.ircmaxell.com

[5] http://fabien.potencier.org

[6] http://lornajane.net

[7] http://philsturgeon.co.uk/blog

[8] http://richardmiller.co.uk

[9] http://phparch.com

[10] https://jtreminio.com