From the PHP_CodeSniffer (phpcs) package information on PEAR:

PHP_CodeSniffer tokenises PHP, JavaScript and CSS files and detects violations of a defined set of coding standards.

This project is a collection of PHP_CodeSniffer rules (sniffs) to validate code developed for WordPress.

This is a fork of the WordPress Coding Standards project from Urban Giraffe published in 2009, at which time Matt Mullenweg gave it a shoutout. A couple years later, the project was picked up by Chris Adams who published it to a repo on GitHub in May 2011. Initially Chris added a missing ruleset.xml file which prevented the rules from being detected by phpcs. Since that time there have been around a dozen contributions to improve the project. It is surprising that there has not been more community involvement in developing these sniffs, as it is a very useful tool to ensure code quality and adherence to coding conventions, especially the official WordPress Coding Standards which are currently only partially accounted for by the sniffs. X-Team has forked the project and is dedicating resources to further develop it and make it even more useful to the WordPress community at large.

The sniffs were developed for phpcs 1.3; work will be done to ensure compatibility with the latest version, which is 1.4.

Ongoing development will be done in the develop with merges done into master once considered stable. Development of unit tests is needed, per issue 21.

Once you've installed PEAR, install Codesniffer:

pear install --alldeps PHP_CodeSniffer

Then install WordPress standards

git clone git:https://github.com/x-team/WordPress-Coding-Standards.git $(pear config-get php_dir)/PHP/CodeSniffer/Standards/WordPress

Normally when working with PEAR, the pear install command is used, but GitHub automatically names the files in a way that will confuse the pear install, so we're falling back to git instead.

Then run the PHP code sniffer commandline tool on a given file, for example wp-cron.php.

phpcs --standard=WordPress -s wp-cron.php

You can use this to sniff individual files, or use different flags to recursively scan all the directories in a project. This command will show you each file it's scanning, and how many errors it's finding:

phpcs -p -s -v --standard=WordPress .

Output will like this:

Registering sniffs in WordPress standard... DONE (11 sniffs registered)
Creating file list... DONE (705 files in queue)
Processing index.php [47 tokens in 31 lines]... DONE in < 1 second (2 errors, 0 warnings)
Processing wp-activate.php [750 tokens in 102 lines]... DONE in < 1 second (47 errors, 2 warnings)
Processing admin-ajax.php [14523 tokens in 1475 lines]... DONE in 2 seconds (449 errors, 44 warnings)
Processing admin-footer.php [183 tokens in 43 lines]... DONE in < 1 second (19 errors, 0 warnings)
Processing admin-functions.php [43 tokens in 16 lines]... DONE in < 1 second (2 errors, 0 warnings)
Processing admin-header.php [1619 tokens in 196 lines]... DONE in < 1 second (110 errors, 1 warnings)
Processing admin-post.php [144 tokens in 33 lines]... DONE in < 1 second (8 errors, 0 warnings)
Processing admin.php [1906 tokens in 238 lines]... DONE in 1 second (128 errors, 1 warnings)
Processing async-upload.php [623 tokens in 70 lines]... DONE in < 1 second (41 errors, 0 warnings)
Processing comment.php [2241 tokens in 289 lines]... DONE in < 1 second (110 errors, 3 warnings)
Processing colors-classic-rtl.css [517 tokens in 1 lines]... DONE in < 1 second (0 errors, 0 warnings)
Processing colors-classic-rtl.dev.css [661 tokens in 79 lines]... DONE in < 1 second (0 errors, 0 warnings)
Processing colors-classic.css ^C

... and so on...

Lots of WordPress's own code doesn't conform to these standards, so running this on your entire codebase will generate lots, and lots of errors.

Instead, try installing the WordPress standard, then invoking it from a project specific codesniffer ruleset instead, like in the supplied example file.

Remove the .example suffix from project.ruleset.xml and run it in your project root, pointing at a given file:

mv project.ruleset.xml.example project.ruleset.xml
phpcs -s -v -p --standard=./project.ruleset.xml a-sample-file.php

A tiny subset of the options available to codesniffer have been used in this example, and there's much more that can be done in a ruleset.xml file. Check the phpcs documentation to see a fully annotated example to build upon.

Check your PATH if it includes new binaries added into the pear directories. You may have to add :/usr/local/php/bin before you can call phpcs on the command line.

Remember that you can see where PEAR is looking for stuff, and putting things, by calling pear config-show. This is how to find where the phpcs binary was added, and where the PEAR library is by default.