**EDIT: I finally figured this out. Scroll down for my self-answered self-accepted answer (green check mark) **

I’m currently using functions.php to redirect https urls to http for a site which currently has no SSL certificate:

function shapeSpace_check_https() { 
if ((!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') || $_SERVER['SERVER_PORT'] == 443) {
    
    return true; 
}
    return false;
}


function bhww_ssl_template_redirect() {
if ( shapeSpace_check_https() ) {

    if ( 0 === strpos( $_SERVER['REQUEST_URI'], 'http' ) ) {
    
        wp_redirect( preg_replace( '|^https://|', 'http://', $_SERVER['REQUEST_URI'] ), 301 );
        exit();
    } else {
            wp_redirect( 'http://' . $_SERVER['HTTP_HOST'] . 
$_SERVER['REQUEST_URI'], 301 );
            exit(); 
        }
    
    }

}

add_action( 'template_redirect', 'bhww_ssl_template_redirect');

In this same function, I’d like to also redirect the www subdomain to non-www. I’ve found a good function here, but need help implementing it into my current function. I’d like to avoid doing this in .htaccess, but would welcome a solution there as well.

How to redirect HTTPS to HTTP and www to non-www URL with .htaccess:

1. First make sure HTTPS is working and valid. It’s easy (and free) to do with Let’s Encrypt these days.

   Note: Although you are redirecting HTTPS to HTTP, I’d recommend doing it the opposite, i.e. HTTP to HTTPS. Better for Security, SEO & Browser compatibility – popular browsers are increasingly making it difficult for HTTP sites.

2. Then make sure .htaccess and mod_rewrite module is working.

3. Then use the following CODE in the .htaccess file of your web root directory (if you are already using some rules there, adjust them accordingly with these new rules):

   <IfModule mod_rewrite.c>
       RewriteEngine On
   
       RewriteCond %{HTTPS}        =on   [OR]
       RewriteCond %{HTTP_HOST}    !^example\.com$
       RewriteRule ^(.*)$          "http://example.com/$1" [R=301,L]
   
       # remaining htaccess mod_rewrite CODE for WordPress
   </IfModule>
   

   Note: Replace example.com with your own domain name.

Why .htaccess solution is better:

It’s better to do these sort of redirects with the web server. From your question, since your web server seems to be Apache, it’s better to do with .htaccess. Why:

1. It’s faster.
2. Your functions.php file remains cleaner & does what it’s originally there for, i.e. Theme modifications.
3. Changing the theme will not affect this.
4. For every redirect, the entire WordPress codebase doesn’t have to load twice – once before redirect & then after redirect.