How do I properly debug and test my code?

Here is what I wrote in the wiki for my company:

Here are some ways to debug when working on the application.

Wp-Config.php

Edit wp-config.php and set WP_DEBUG to true. It is default to false. It will look like this when debug is set to true.

/**
 * For developers: WordPress debugging mode.
 *
 * Change this to true to enable the display of notices during development.
 * It is strongly recommended that plugin and theme developers use WP_DEBUG
 * in their development environments.
 *
 * For information on other constants that can be used for debugging,
 * visit the Codex.
 *
 * @link https://codex.wordpress.org/Debugging_in_WordPress
 */
define('WP_DEBUG', true);

PHP error log

You can stream the log live by going to the error log location and tailing the log. This will work on Apache setups.

cd /var/log/apache2 && tail -f error.log

PHP Code

The var_dump function displays structured information about variables/expressions including its type and value. Arrays are explored recursively with values indented to show structure. It also shows which array values and object properties are references.

The print_r() displays information about a variable in a way that's readable by humans. array values will be presented in a format that shows keys and elements. Similar notation is used for objects.

var_dump($expression);

and

print_r($expression);

Benchmarking Speed

// start timer
$time = -microtime(true);

// hello world function
function test_123() {
  echo "hello world";
}

// function to check
test_123();

// end timer
$time += microtime(true);

// display benchmark
echo sprintf('%f', $time),PHP_EOL;

Javascript console

You can dump into the console when coding javascript

console.log(var);

Plugins

Deactivate plugins and use process of elimination to figure out which one is causing the issue.