polish_notation.php

<?php 
  $polish_notation = "+ 9 * 2 6"; //space seperated polish notation
  $invalid_polish_notation = "1 + 2";
  $answer = polish_notation_calculator($polish_notation);
  print_r($answer."\n");
  
  $answer = polish_notation_calculator($invalid_polish_notation);
  print_r($answer);
  
  function polish_notation_calculator($polish_notation) {
    $operator_stack = new Stack();
    $operand_stack = new Stack();
    $pending_operand = false;
    $polish_notation_arr = explode(" ",$polish_notation);
    
    
    for($i=0; $i < count($polish_notation_arr); $i++) {
      $char = $polish_notation_arr[$i];
      if ($char == "+" || $char == "-" || $char == "/" || $char == "*") {
        $operator_stack->push($char);
    
        $pending_operand = false;
      }else if(is_numeric($char)){
        $operand = $char;
        if($pending_operand == true) {
          while(!$operand_stack->is_empty()) {
            $operand1 = $operand_stack->pop();
            $operator = $operator_stack->pop();
    
            if ($operator == "+") $operand = $operand1 + $operand;
            else if ($operator == "-") $operand = $operand1 - $operand;
            else if ($operator == "*") $operand = $operand1 * $operand;
            else if ($operator == "/") $operand = $operand1 / $operand;
          }
        }
        $operand_stack->push($operand);
        $pending_operand = true;
      }
    }
    if ($operator_stack->is_empty() || count($operand_stack)>1)
      return $operand_stack->pop();
    else 
      return "Not a valid polish notation.";
}
?>


<?php 
  class Stack
  {
    
    public $stack;
    
    public function __construct() {
      $this->stack = array();
    }
    
    public function push($x) {
      array_unshift($this->stack,$x);
    }
    
    public function pop() {
      if ($this->is_empty()) {
        throw new RuntimeException("Stack is empty!!! No pop operation can be performed.");
      }else {
        return array_shift($this->stack);
      }
    }
    
    public function top() {
      return current($this->stack);
    }
    
    public function is_empty() {
      return empty($this->stack);
    }
  }
?>