alert-box-shortcode.php

<?php
/*
Author: WP Sharks
Author URI: https://wpsharks.com
Version: 1.0
Plugin Name: Alert Box Shortcode
Description: Provides an `[alert_box]` shortcode.
Plugin URI: https://wordpress.org/plugins/alert-box-shortcode/
Text Domain: alert-box-shortcode
*/

add_action('plugins_loaded', function () {
    add_shortcode('alert_box', 'alert_box');
});

function alert_box($attr, $content, $shortcode)
{
	static $counter = 0;
	$id = 'alert-box-'.$counter;
	$short_code_content;

	$attr = shortcode_atts(
	    [
	    	'border_width'     => '1px',
	    	'background_color' => '#ffe6e6',
	    	'border_color'     => '#ff8080',
	    	'text_color'       => '#000000',
	    	'link_color'       => '#b30000',
	    	'link_hover_color' => '#4d0000',
	    	'border_radius'    => '.25em',
	    	'padding'          => '1em',
	    	'style'            => ''
	    ],
	    $attr,
	    $shortcode
	);

	$css = '
	<style type="text/css">
	#'.$id.' {
	    border-width: '.$attr['border_width'].';
	    background-color: '.$attr['background_color'].';
	    border-radius: '.$attr['border_radius'].';
	    padding: '.$attr['padding'].';
	    color: '.$attr['text_color'].';
	}

	#'.$id.' a {
	    color: '.$attr['link_color'].';
	}

	#'.$id.' a:hover {
	    color: '.$attr['link_hover_color'].';
	}

	</style>
	';

	$counter++;

	$style = "";

	if( isset( $attr[ 'style' ] ) )
	{
		$style = 'style="'.$attr[ 'style' ] . '"';
	}

	$short_code_content = $css; // Styles from above.

	$short_code_content .= '<div id="'.esc_attr($id).'" '. $style .'>';
	$short_code_content .=    trim($content);
	$short_code_content .= '</div>';

	return $short_code_content;
}