A nice wp_debug_var() as a mu-plugin to include in your projects

wp_debug_var.php

<?php
/*
Plugin Name: WP Debug Function
Description: A Must-Use plugin that makes the wp_debug_var available to your WordPress Installation
Plugin URI:  https://gist.github.com/broskees/18c6b10a491d2946fbbfed96a8dd6dc4
Version:     1.0.0
Author:      Joseph Roberts
Author URI:  https://github.com/broskees
License:     GPLv2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
*/

if (!defined( 'ABSPATH' )) {
    exit; // Exit if accessed directly
}

class WP_DEBUG_VAR
{
    private $vars;
    private $wp_backtrace;
    private $output;

    public function __construct($vars)
    {
        $this->vars = array_filter(array_map(
            function ($var) {
                return htmlspecialchars(print_r($var, true));
            },
            $vars
        ));

        $this->wp_backtrace = print_r(wp_debug_backtrace_summary(null, 2, false), true);
    }

    public function printOutputAndLive()
    {
        echo $this->getNotDieOutput();
    }

    public function printOutputAndDie()
    {
        wp_die($this->getOutput());
    }

    private function getOutput($live = false)
    {
        ob_start();
        ?>
            <style>
                .tab_content {
                    <?php if (!$live): ?>
                        max-height: calc(100vh - 1em - 170px); /* Make it fix within wp_die message nicely */
                    <?php else: ?>
                        max-height: 237px;
                    <?php endif; ?>
                    overflow-y: scroll;
                    display: none;
                }
                #tab_1:checked ~ #content_1,
                #tab_2:checked ~ #content_2,
                #tab_3:checked ~ #content_3 { display: block; }
            </style>

            <div>
                <input type="radio" name="tab_check" class="tab_check" id="tab_1" checked>
                <label for="tab_1">Var Output</label>
                <input type="radio" name="tab_check" class="tab_check" id="tab_2">
                <label for="tab_2">WP Backtrace</label>
                <input type="radio" name="tab_check" class="tab_check" id="tab_3">
                <label for="tab_3">PHP Backtrace</label>
                <div class="tab_content" id="content_1">
                    <?php foreach($this->vars as $var): ?>
                        <pre><?php echo !empty($var) ? $var : '' ?></pre>
                    <?php endforeach; ?>
                </div>
                <div class="tab_content" id="content_2">
                    <pre><?php echo !empty($this->wp_backtrace) ? $this->wp_backtrace : '' ?></pre>
                </div>
                <div class="tab_content" id="content_3">
                    <pre><?php debug_print_backtrace() ?></pre>
                </div>
            </div>
        <?php
        return ob_get_clean();
    }

    private function getNotDieOutput()
    {
        ob_start();
        ?>
            <style>
                .wp_debug_var_modal {
                    position: fixed;
                    height: 300px;
                    width: 100vw;
                    bottom: 0;
                    background-color: #fff;
                    padding: 20px;
                    z-index: 9999999999;
                }
            </style>
            <div class="wp_debug_var_modal">
                <?= $this->getOutput(true) ?>
            </div>
        <?php
        return ob_get_clean();
    }
}

/**
 * Debugging function
 *
 * @param mixed $variable
 */
function wp_debug_var(...$vars) {
    if (apply_filters('wps_debug_off', false)) {
        return;
    }

    (new WP_DEBUG_VAR($vars))->printOutputAndDie();
}

/**
 * Debug and don't die
 * 
 * @param mixed $variable
 */
function wp_debug_var_live(...$vars) {
    if (!current_user_can('manage_options')) {
        return;
    }

    add_filter('wp_debug_vars', function ($already_set_vars) use ($vars) {
        if (!empty($vars)) {
            $already_set_vars = array_merge_recursive($already_set_vars, $vars);
        }

        return $already_set_vars;
    });

    $callback = function () {
        (new WP_DEBUG_VAR(
            apply_filters('wp_debug_vars', [])
        ))->printOutputAndLive();
    };
    
    if (!has_action('shutdown', $callback)) {
        add_action('shutdown', $callback);
    }
}