PHP has been around for 21 years now, since the first version of the language appeared in 1995. During this long period, the language went through a lot of changes and improvements and has provided many developers with valuable experience.
In this article, you will find four valuable resources of coding standards and best practices.
PHP: The Right Way
PHP: The Right Way is an easy-to-read, quick reference for PHP popular coding standards, links to authoritative tutorials around the Web and what the contributors consider to be the best practices at the present time.
PHP Best Practices
PHP Best Practices is a guide suggesting the best direction to take when facing one of the common low-level tasks a PHP programmer might encounter that are unclear because of the many options PHP might offer. For example: connecting to a database is a common task with a large amount of possible solutions in PHP, not all of them are good ones—thus, it’s included in this document.
TIM - PHP Best Practices
There is a number of good practices that you should follow when developing web applications in PHP. Most of these are extremely easy to pick up and some of them will even apply to web application development in general. Here you can find 35 very important PHP practices.
Last one we're mentioning here is php.net, a valuable resource of news, documentation and another information about PHP.
Fun fact: Similarly to Linux, PHP also uses an animal as its mascot with the added pun play on the word as a bonus: elePHPant. The creator of the logo (it's not the official PHP logo, though) with the elephant says that if you look at the capital letters PHP from what he called "sideways angle" they form a shape of an elephant. Neat, isn't it?