Wordpress 3rd party depency checks. Verify that a specific theme, child theme or plugin is installed and active. This snippet checks wheter the Divi theme, Divi child theme or Divi Builder plugin is installed.

divicheck.php

<?php 
/**
* @version 1.0.0
* @author Theuns Coetzee (pokkel/ipokkel)
* @link       http://www.theunscoetzee.com
*/

/**
* @param checkstatus( string, bool )
* Accepts string and boolean argument
* Returns string in green when boolean true and red when false
*/
function checkstatus($checked_item, $checked_status) {
    $the_colour = 'red';
    if ($checked_status) {
        $the_colour = 'green';
    }
    $the_phrase = '<div style="color:' . $the_colour . '"><p>' . $checked_item . '</p></div>';
    return $the_phrase;
}

function get_default_theme_stylesheet_path(){
    $the_theme_name = wp_get_theme()->get_core_default_theme()->get( 'TextDomain' );    
    $the_stylesheet_path = get_theme_root() . '/' .  $the_theme_name . '/style.css'; 
    return str_replace('\\', '/', $the_stylesheet_path);
}

function get_active_theme_stylesheet_path(){   
    $the_stylesheet_path = wp_get_theme()->get_stylesheet_directory() . '/style.css';
    return str_replace('\\', '/', $the_stylesheet_path);
}

function get_custom_theme_stylesheet_path($the_theme_name){
    $the_stylesheet_path = get_theme_root() . '/' .  $the_theme_name . '/style.css';
    return str_replace('\\', '/', $the_stylesheet_path);
}

function validate_theme_stylesheet($the_theme_name) {
    if ( ! file_exists( get_custom_theme_stylesheet_path($the_theme_name) ) ) {
        // Invalid.
    } else {
        // Valid.
        return true;
    }
}

    // Check if the divi theme exist in the themes folder.
    echo checkstatus('Divi theme exist', validate_theme_stylesheet('Divi')); // Done

    // Check if Divi is the active theme
    $theme = wp_get_theme( );
    echo checkstatus('Divi is active', ( wp_get_theme( )->name == 'Divi'));

    // Check if Current theme has Divi as parent
    echo checkstatus('Divi is parent', ( wp_get_theme()->template == 'Divi'));

    // Check whether divi builder installed
    echo checkstatus('Builder is active', (is_plugin_active('divi-builder/divi-builder.php')));

    $plugin_root = str_replace('\\', '/', WP_PLUGIN_DIR) . '/'; // convert backslash to forward slash in string when on windows. This can be removed for the stable/production version. 
    $Divi_Builder_Plugin_Path = $plugin_root . 'divi-builder/divi-builder.php';  // construct path string
    echo checkstatus('plugin exist in directory but is not installed or active', (file_exists($Divi_Builder_Plugin_Path)));

    // Check whether divi builder active
    echo checkstatus('Divi Builder Plugin class exist', (class_exists('ET_Builder_Plugin')));

?>