Accessing post data before functions.php in Wordpress - php

Question

I am updating my custom Wordpress theme, but as it is rather time consuming process I would like to launch one section at a time.
In other words there would be several versions of a theme for different parts of the website.
To keep things tidy I would like to keep them in separate folders, with all the assets such as js, images, css.
I managed to rewrite template hierarchy using conditional tags but got stuck on functions.php
I was trying to use custom filed (post meta) to switch between several functions.php files but unfortunately $post is not available there so I am unable to use get_post_meta().
I could only find a trace of a solution with custom db queries, $wpdb etc. but can't really figure it out.
Is there any fairly simple solution to hook-up into post data (wp_query) before functions.php is loaded? Or to somehow differently modify where the functions are loaded from?
To illustrate what I'm writing about I pasted my main index.php
<?
get_header();
/*
* Get theme version according to the custom field 'section'
*/
if( function_exists ( 'theme_version' ) ){
$theme = theme_version( #get_the_ID() );
} else {
$theme = 'v2';
}
include_once( 'theme/'. $theme .'/TEMPLATE_BUILDER.php' );
get_footer();
?>
Thanks!

Answer 1

Hopefully found a right answer (after couple of hours of research, trial and error)
I placed the below code in main (wp-native) functions.php
in attempt to keep the code and file structure tidy, works as a charm.
add_action('after_setup_theme', function(){
// parse_url tidies-up the uri
$section = get_post_meta( url_to_postid( parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH ) ),'section', true);
if ( !empty( $section )){
// assign sections to the theme versions below
$theme_version = array(
'v3' => array(
'Sixth Form',
[ ... ]
),
'v3.1' => array(
'Contact',
[ ... ]
)
);
foreach($theme_version as $key => $value) { if(in_array( $section, $value )) $theme = $key; }
}
if( empty($theme) ) $theme = 'v2'; // default theme version
require_once( 'theme/' . $theme . '/functions.php' );
$GLOBALS['theme-ver'] = $theme; // set the global to use in index.php (and somewhere else perhaps)
});
The code is not complete yet – needs some conditional clauses as the functions.php is sometimes called multiple times within the loop (especially with custom wp_query)
Perhaps someone will find the above useful. By the way it's quite surprising that WP doesn't natively support some kind of 'theme version control' – I can see strong benefits of not having to upgrade the whole website at once for instance to e. g. resp.