Coding style (programming style) is a set of rules and guidelines how to indent code, where to put opening curly bracket, use camelCase, StudlyCase, or under_score conventions and similar.
PHP allows a lot of code styles, which is good and very friendly to start with. However, as soon as you write more PHP code or when collaborating on PHP code with others, different coding styles will get in a way very soon and code will be less readable.
For PHP, there are already existing standards and coding style recommendations that you should look into when writing code. Adopting a consistent coding style will simplify collaboration on code and make it more readable.
One of the most adopted standards recommendations in PHP is PHP FIG, which recommends coding style via two of its so called PSRs (PHP Standard Recommendation):
Many open source projects also extend above PSRs with their own coding style guides. For example:
Many advanced PHP IDEs and editors also offer code refactoring via plugins and extensions. With predefined common coding styles such as PSR-1 and PSR-2, with refactoring code, automatic code generation, autocomplete and similar things, consistently writing standardized PHP code can be done with ease.
The more you write code, the more you will understand the importance in using common style guides. Especially when working with source code control (eg, Git, coworkers), or just to be consistent in general.
These are some further best practices that are either not defined in the PSR coding style yet, or are recommended by the PHP documentation.
Constants should be defined with uppercase letters and underscore separators:
<?php const FOO = 'value'; const FOO_BAR = 'value';
__FOO__, because PHP might one day use such contant name internally.
Some of the PHP open source projects and their coding styles: