This MU-Plugin helps preset or preselect a country/state based on checkout. It can be shipping or billing. All you need to do is call the function 'CLASS::process_selected_country_or_state_etc('NG')' to set the value. Inspired by https://wordpress.org/support/topic/change-session-shipping-country-on-product-page-collection-page-2/ . Use at your …

pekky-wc-change-session-country-state.php

<?php
/**
 * WooCommerce Change Session Country/State for checkout.
 *
 * @package pekky-wc-change-session-country-state
 * @author Precious Omonzejele (CodeXplorer 🤾🏽‍♂️🥞🦜🤡)
 *
 * @wordpress-plugin
 * Plugin Name: WooCommerce Change Session Country/State for checkout
 * Plugin URI: https://gist.github.com/Preciousomonze/58a1e85557a74a4094f245adb0f8786f
 * Description: This MU-Plugin helps preset or preselect a country/state based on checkout. It can be shipping or billing. All you need to do is call the function 'CLASS::process_selected_country_or_state_etc('NG')' to set the value.
 * Author: Precious Omonzejele (CodeXplorer 🤾🏽‍♂️🥞🦜🤡)
 * Author URI: https://codexplorer.ninja
 * Version: 1.0.0
 * Requires at least: 5.0
 * Tested up to: 5.9
 * WC requires at least: 5.0
 * WC tested up to: 6.0
 */
class Pekky_WC_Change_Session_Country_State {

	/**
	 *  Default country. (e.g: NG for Nigeria).
	 *
	 * @var string
	 */
	protected static $wc_default_country = '';

	/**
	 * If the override/change should affect logged in Customers.
	 *
	 * @var bool
	 */
	public static $affect_existing_customers = false;

	/**
	 * Type.
	 *
	 * state or country.
	 * 
	 * @var string
	 */
	protected static $type = '';

	/**
	 * Boostrap and shoot.
	 */
	public static function init() {
	
		// Hook on checkout.
		add_filter( 'woocommerce_before_checkout_form', array( __CLASS__, 'set_selected_country_or_state_etc' ), 10 );

	}

	/**
	 * Check if woocommerce is active.
	 */
	public function is_woocommerce_active() {
		if ( class_exists( 'woocommerce' ) ) { 
			return true; 
		} else { 
			return false; 
		}
	}

	/**
	 * Processes Selected country or state.
	 *
	 * ETC is incase we later support more values like city.
	 * 
	 * @param string $value Value should be the abbr of the country or state, e.g NG for Nigeria.
	 * @param string $type (optional) any valid type like country,state,town. only country and state supported for now.
	 */
	public static function process_selected_country_or_state_etc( $value, $type = 'country' ) {
		// If woocommerce isn't active or empty data, do nothing.
		if ( !self::is_woocommerce_active() || empty( $value ) ) {
			return;
		}

		if ( is_null( WC()->session ) ) {
			WC()->session = new WC_Session_Handler();
			WC()->session->init();
		}

		// Start storing.
		$process_type = strtolower( $type );
		self::$type   = $process_type;

		/**
		 * Filters the session name for the selected country or state.
		 */
		$session_name = apply_filters( 'pekky_wc_csc_selected_' . $process_type . '_session_name', 'pekky_selected_' . $process_type );

		WC()->session->set( $session_name, $value );
	}

	/**
	 * Set selected country or state.
	 *
	 * ETC is incase we later support more values like city.
	 * 
	 * @hooked
	 */
	public static function set_selected_country_or_state_etc() {
		  // If we set affect_existing_customers to false and user already exists, don't override country.
		  if ( ! self::$affect_existing_customers &&  WC()->customer->get_is_paying_customer() ) {
			return;
		}

		/**
		 * Filters the field type being changed.
		 */
		$process_type = strtolower( apply_filters( 'pekky_wc_csc_field_type', self::$type ) );

		/**
		 * Filters the session name for the selected country or state.
		 */
		$session_name = apply_filters( 'pekky_wc_csc_selected_' . $process_type . '_session_name', 'pekky_selected_' . $process_type );

		$selected = WC()->session->get( $session_name );

		if ( ! empty( $selected ) ) {
			/**
			 * Filters the country type country.
			 * 
			 * Values can only be billing or shipping. Default is shipping.
			 */
			$country_state_etc_type = strtolower( apply_filters( 'pekky_wc_csc_country_state_etc_type', 'shipping' ) );

			// ..customer->set_billing_country.
			WC()->customer->{"set_" . $country_state_etc_type . "_" . $process_type}( $selected );
		}

	}

}

// Launch!
Pekky_WC_Change_Session_Country_State::init();