Vjylku

Is there an easy and standard way to debug our code in wordpress plugins?
I want to print some variables in a function that runs when a certain things happen.
Is there a wordpress way to save this in a file or at least see it?

Just start with the following definitions in your config.php file:

define('WP_DEBUG', true);

define('WP_DEBUG_LOG', true);

define('SCRIPT_DEBUG', true);

define('SAVEQUERIES', true);

The first one sets your system into debug mode, the second one writes every notice into a log file saved into your wp-content directory. Script debug enables WordPress to use the uncompressed CSS and JS files (Maybe also interesting for writing Plugins). And Savequeries writes HTML-Comments at the bottom of the rendered HTML Output and will be used by Plugins I suggest you next.

See the WordPress Plugin Directory for Debug Bar and extensions. There are a lot and there's a great chance that some of them are providing exactly what you need.

If nothing helps you can write everytime

print_r( $var );

die();

and look into the HTML sourcecode.

Unfortunately there isn't really a standard developer logging API in WordPress. If you want to output your own data to a debug file (to prevent "headers already sent" errors) then you can simply use the standard PHP error_log function. Something simple like this will record the variable $data to the file debug.log in your plugins directory:

$debugline = '['.date('y m d H:i:s', time()).'] '.print_r($data,true);

$debugfilepath = dirname(__FILE__).'/debug.log';

error_log($debugline, 3, $debugfilepath); 

The best way to troubleshoot/debug your PHP code for WordPress is to use the XDEBUG debugger for PHP coupled with an IDE/editor that can leverage XDEBUG.

By far my favorite IDE for PHP/XDEBUG is PhpStorm (best money I have ever spent), but you can use for free on the bleeding edge with their EAP (Early Access Program). You can also use the free VSCode with XDEBUG.

Here is a great video showing what it looks like to use XDEBUG & PhpStorm. You can start at the 18:00 point because everything before that is talking and slides.

You will also need a local development setup that supports XDEBUG. Several years ago I was frustrated with how difficult it was to get XDEBUG configured correctly so I had my team develop WPLib Box which pre-configures XDEBUG and thus makes using XDEBUG trivally easy. You can download WPLib Box from GitHub, use for free, and get help via our Slack.

I like to say using PhpStorm/XDEBUG/WPlib Box gives me WordPress superpowers. How else do you think I answered all the questions here at WPSE that I have? :-)

That said, there are also other local development solutions such as VVV, Trellis, Chassis, Desktop Server, MAMP, DevilBox, LaraDock, Kalabox, Kusanagi and others you might try. But I am not sure how difficult it is to set up XDEBUG to work with any of these others anymore and I know firsthand XDEBUG works without any fuss when using WPLib Box.

Hope this helps.