This is a simple currency converter. The list of allowed currencies and exchange rates are stored in a single database table with the following fields: currency_id (int), currency_code (3 letter string), exchange_rate (float, stored as the rate from US dollars to the given currency)

PHP and CI currency converter

/* ==========================================================================
   Model
============================================================================= */
 
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
  class Exchange_calculator_model extends CI_Model {
  	public function __construct() {
			$this->load->database();
		}
 
		public function get_exchg_rate($currency = NULL) {
			$error = "That code does not exist or is in an incorrect format.";
			
			// check that input contains only alphabetic characters
			if (ctype_alpha($currency) && strlen($clean_currency) == 3) {
				
				// use Active Record to run query
				$this->db->select('exchange_rate');
				$this->db->from('exchange_rates');
				$this->db->where('currency_code', $clean_currency);
				
				$query = $this->db->get();
				
				// check that we got results
				if ($query->num_rows() > 0) {
					foreach ($query->result() as $row) {
					  $exchg_rate = $row->rate;
					}
				} else {
					return $error;
				}
 
				return $exchg_rate;
 
			} else {
				return $error;
			}
		}
	}
?>
 
/* ==========================================================================
   Controller
============================================================================= */
 
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 
class Exchange_calculator extends CI_Controller {
	public function __construct() {
		$this->load->model('exchange_calculator_model');
	}
	
	// checks to make sure the returned value from the model is not an error
	private function return_error($exchg_rate) {
		if (is_float($exchg_rate) || is_int($exchg_rate)) {
			return;
		} else {
			// in this case, $exchg_rate will be the error from the model, and it is sent to the user
			echo $exchg_rate;
		}
	}
		
	public function convert_currency($currencyFrom, $currencyTo, $amount) {
		$exchg_rate;
		$converted;
		$from_exchg_rate;
		$to_exchg_rate;
		$usd;
 
		// if exchanging from USD, get exchange rate and multiply by amount
		if ($currencyFrom == “USD”) {
			$exchg_rate = $this->exchange_calculator_model->get_exchg_rate($currencyTo);
			$this->return_error($exchg_rate);
			$converted = $amount * $exchg_rate;
		} elseif ($currencyTo == “USD”) {
			// if exchanging to USD, get exchange rate and divide amount by it
			$exchg_rate = $this->exchange_calculator_model->get_exchg_rate($currencyFrom);
			$this->return_error($exchg_rate);
			$converted = $amount / $exchg_rate;
		} else {
			// if neither currency is USD, convert to USD and multiply by exchange rate since we only have exchange rates to USD
			$from_exchg_rate = $this->exchange_calculator_model->get_exchg_rate($currencyFrom);
			$this->return_error($from_exchg_rate);
			$to_exchg_rate = $this->exchange_calculator_model->get_exchg_rate($currencyTo);
			$this->return_error($to_exchg_rate);
 
			$usd = $amount / $from_exchg_rate;
			$converted = $usd * $to_exchg_rate;
		}
 
		return $converted;
	}
}
?>
 
/* 
*  ExchangeCalculator.convert_currency(“USD”, “PHP”, 100) should return 4312
*  ExchangeCalculator.convert_currency(“PHP”, “USD”, 100) should return 2.32
*  
*  If LEU has hypothetical exchange rate to USD of 10:
*  ExchangeCalculator.convert_currency(“PHP”, “LEU”, 100) should return 23.19
*  ExchangeCalculator.convert_currency(“LEU”, “PHP”, 100) should return 431.23 
*/