Task: Parsing_RPN calculator algorithm
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Globalization;
using System.Threading;

namespace RPNEvaluator
{
    class RPNEvaluator
    {
        static void Main(string[] args)
        {
            Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;

            string rpn = "3 4 2 * 1 5 - 2 3 ^ ^ / +";
            Console.WriteLine("{0}\n", rpn);

            decimal result = CalculateRPN(rpn);
            Console.WriteLine("\nResult is {0}", result);
        }

        static decimal CalculateRPN(string rpn)
        {
            string[] rpnTokens = rpn.Split(' ');
            Stack<decimal> stack = new Stack<decimal>();
            decimal number = decimal.Zero;

            foreach (string token in rpnTokens)
            {
                if (decimal.TryParse(token, out number))
                {
                    stack.Push(number);
                }
                else
                {
                    switch (token)
                    {
                        case "^":
                        case "pow":
                            {
                                number = stack.Pop();
                                stack.Push((decimal)Math.Pow((double)stack.Pop(), (double)number));
                                break;
                            }
                        case "ln":
                            {
                                stack.Push((decimal)Math.Log((double)stack.Pop(), Math.E));
                                break;
                            }
                        case "sqrt":
                            {
                                stack.Push((decimal)Math.Sqrt((double)stack.Pop()));
                                break;
                            }
                        case "*":
                            {
                                stack.Push(stack.Pop() * stack.Pop());
                                break;
                            }
                        case "/":
                            {
                                number = stack.Pop();
                                stack.Push(stack.Pop() / number);
                                break;
                            }
                        case "+":
                            {
                                stack.Push(stack.Pop() + stack.Pop());
                                break;
                            }
                        case "-":
                            {
                                number = stack.Pop();
                                stack.Push(stack.Pop() - number);
                                break;
                            }
                        default:
                            Console.WriteLine("Error in CalculateRPN(string) Method!");
                            break;
                    }
                }
                PrintState(stack);
            }

            return stack.Pop();
        }

        static void PrintState(Stack<decimal> stack)
        {
            decimal[] arr = stack.ToArray();

            for (int i = arr.Length - 1; i >= 0; i--)
            {
                Console.Write("{0,-8:F3}", arr[i]);
            }
            
            Console.WriteLine();
        }
    }
}

PHP: <?php
function rpn($postFix){
    $stack = Array();
    echo "Input\tOperation\tStack\tafter\n" ;
	$token = explode(" ", trim($postFix));
	$count = count($token);
	
    for($i = 0 ; $i<$count;$i++)
	{
        echo $token[$i] ." \t";
        $tokenNum = "";
		
        if (is_numeric($token[$i])) {
            echo  "Push";
			array_push($stack,$token[$i]);
        }
        else
        {
            echo "Operate";
            $secondOperand = end($stack);
			array_pop($stack);
            $firstOperand = end($stack);
            array_pop($stack);

            if ($token[$i] == "*")
				array_push($stack,$firstOperand * $secondOperand);
            else if ($token[$i] == "/")
                array_push($stack,$firstOperand / $secondOperand);
            else if ($token[$i] == "-")
                array_push($stack,$firstOperand - $secondOperand);
            else if ($token[$i] == "+")
                array_push($stack,$firstOperand + $secondOperand);
            else if ($token[$i] == "^")
                array_push($stack,pow($firstOperand,$secondOperand));
            else {
                die("Error");
            }
        }
		echo "\t\t" . implode(" ", $stack) .  "\n";
    }
    return end($stack);
} 

echo "Compute Value: " . rpn("3 4 2 * 1 5 - 2 3 ^ ^ / + ");
?>