Currency switcher thingy

currency-thingy.php

<?php
/*
Plugin name: Currency switcher thingy
*/

add_action( 'plugins_loaded', 'currency_setup' );

function currency_setup() {
	add_action( 'publish_post', 'currency_publish_post' );
}

function currency_publish_post( $post_id ) {
	//Get the price and currency custom fields
	$price = get_post_meta( $post_id, 'price', true );
	$currency = get_post_meta( $post_id, 'currency', true );

	//If the custom fields aren't empty
	if ( ! empty( $price ) && ! empty( $currency ) ) {
		//Work out the prices in different currencies
		$G = currency_exchange( $currency, 'GBP', $price );
		$E = currency_exchange( $currency, 'EUR', $price );
		$U = currency_exchange( $currency, 'USD', $price );

		//Add the exchanged prices to post meta, if the currency exchange was successful
		//Using update_post_meta instead of add_post_meta, so values are updated on each save, rather than new values added

		if ( false !== $G ) {
			update_post_meta( $post_id, 'GBPprice', $G );
		}

		if ( false !== $E ) {
			update_post_meta( $post_id, 'EURprice', $E );
		}

		if ( false !== $G ) {
			update_post_meta( $post_id, 'USDprice', $U );
		}
	}
}

function currency_exchange( $from, $to, $amount ) {
	//Retrieve the currency exchange rates
	$request = wp_remote_get( 'http://openexchangerates.org/latest.json' );

	//If the remote request didn't return an error
	if ( ! is_wp_error( $request ) && 200 == $request['response']['code'] ) {
		//Response is json format, so decode it into an array
		$rates = json_decode( $request['body'], true );

		//If the json decoding was successful
		if ( false !== $rates ) {
			//Make sure the $from and $to parameter are strings
			$from = (string) $from;
			$to = (string) $to;

			//Make sure the $amount parameter is a float
			$amount = (float) $amount;

			//If the $from and $to currencies exist in the array, work out the $amount in the $to currency and return it
			if ( isset( $rates['rates'][$from] ) && isset( $rates['rates'][$to] ) ) {
				return $amount * ( $rates['rates'][$to] * ( 1 / $rates['rates'][$from] ) );
			} else {
				return false; //One or both of the specified currencies don't exist in the array
			}
		} else {
			return false; //json_decode failed, perhaps invalid json
		}
	} else {
		return false; //An error occurred when retrieving the data
	}
}