1.php

<?php
if (!version_compare(PHP_VERSION, '5.6.0', '>=')) {
    function slug_php_version_nag()
    {
        ?>
        <div class="notice notice-error">
            <p>
                <?php _e('Your version of PHP is incompatible with YOUR PLUGIN and can not be used.', 'caldera-forms'); ?>
                <?php printf(' <a href="https://your-plugin.com/php?utm_source=wp-admin&utm_campaign=php_deprecated&utm_source=admin-nag" target="__blank">%s</a>',
                    esc_html__('Learn More', 'slug')
                ) ?></p>
        </div>
        <?php
    }

    add_action('admin_notices', 'slug_php_version_nag');
} 

2.txt

=== Your plugin ===
Contributors: Shelob9
Tags: hi-oy
Requires at least: 4.7
Tested up to: 4.9.7
Stable tag: 1.7.2
License: GPLv2
Requires PHP: 5.6

Hey, Josh. I saw this gist embedded at https://torquemag.io/2018/08/how-to-drop-legacy-php-support-in-wordpress-plugins/

For those who copy/paste without reading first, please change that License to add or any later version -- better yet, GPL version 3 or any later version