Anyone who has done a bit of WordPress programming knows that solid knowledge of PHP is required to develop great plugins and themes. Even if you are not planning to release your own theme or plugin, you should still know a thing or two about PHP to be able to fix theme and plugin issues. There are many books and online courses you can take to take your PHP knowledge to the next level. These 6 video and interactive sites help you learn PHP faster:
Lynda.com: a membership to this site is not cheap but I have been a member for a few years. They add new material to their website all the time. Lynda.com covers PHP, CSS, HTML, and many other technologies you need to be familiar with.
Udemy: this is a marketplace for tutorials on all kinds of topics. Whether you want to learn how to write a Kindle book, develop great PHP code, or master 3D printing, there is a course available for you here.
Team Tree House: these folks teach you how to write your own apps and learn technologies such as PHP interactively. You receive video-based training along the way.
Code Academy: one of my favorite websites to learn and teach coding. It offers basic coding exercises to help you learn things in a more pragmatic fashion.
Programmr: this program offers live PHP exercises, examples, and projects to take your skills to the next level. Other technologies such as Python, C#, JS, and iOS/Android are covered.
W3 Schools: one of my go-to sites when I am confused about a technology I am using. You can learn the basics and more advanced PHP topics on this site.
These courses make it easier to learn PHP and take your skills to the next level. You are still going to have to work hard to master this technology.