Evaluate postfix expression using stack in php

Php program for Evaluate postfix expression using stack. Here problem description and explanation.

<?php
/* 
    Php program for 
    Evaluate postfix expression
*/
// Stack Node
class StackNode
{
	public $operand;
	public $next;
	public	function __construct($operand, $top)
	{
		$this->operand = $operand;
		$this->next = $top;
	}
}
class MyStack
{
	public $top;
	public $count;
	public	function __construct()
	{
		$this->top = NULL;
		$this->count = 0;
	}
	// Returns the number of element in stack
	public	function isSize()
	{
		return $this->count;
	}
	public	function isEmpty()
	{
		if ($this->isSize() > 0)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Add a new element in stack
	public	function push($value)
	{
		// Make a new stack node
		// And set as top
		$this->top = new StackNode($value, $this->top);
		// Increase node value
		$this->count++;
	}
	// Remove a top element 
	public	function pop()
	{
		$value = $this->peek();
		if ($this->isEmpty() == false)
		{
			$this->top = $this->top->next;
			// Reduce the size
			$this->count--;
		}
		return $value;
	}
	// Used to get top element of stack
	public	function peek()
	{
		if (!$this->isEmpty())
		{
			return $this->top->operand;
		}
		else
		{
			return 0;
		}
	}
}
class Execution
{
	public	function evaluate($e)
	{
		// Get the length of expression
		$size = strlen($e);
		$a = 0;
		$b = 0;
		$s = new MyStack();
		$isVaild = true;
		// work with (+,-,/,*,%) operator
		for ($i = 0; $i < $size && $isVaild; $i++)
		{
			if ($e[$i] >= '0' && $e[$i] <= '9')
			{
				$a = ord($e[$i]) - ord('0');
				$s->push($a);
			}
			else if ($s->isSize() > 1)
			{
				$a = $s->pop();
				$b = $s->pop();
				// Perform arithmetic operations between 2 operands
				if ($e[$i] == '+')
				{
					$s->push($b + $a);
				}
				else if ($e[$i] == '-')
				{
					$s->push($b - $a);
				}
				else if ($e[$i] == '*')
				{
					$s->push($b * $a);
				}
				else if ($e[$i] == '/')
				{
					$s->push((int)($b / $a));
				}
				else if ($e[$i] == '%')
				{
					$s->push($b % $a);
				}
				else
				{
					// When use other operator
					$isVaild = false;
				}
			}
			else if ($s->isSize() == 1)
			{
				// Special case  
				// When use +, - at the beginning
				if ($e[$i] == '-')
				{
					$a = $s->pop();
					$s->push(-$a);
				}
				else if ($e[$i] != '+')
				{
					// When not use  +,-
					$isVaild = false;
				}
			}
			else
			{
				$isVaild = false;
			}
		}
		if ($isVaild == false)
		{
			// Possible case use other operators
			// 1) When using special operators
			// 2) Or expression is invalid
			echo $e.
			" Invalid expression ", "\n";
			return;
		}
		echo $e.
		" = ".strval($s->pop()), "\n";
	}
	public static
	function main($args)
	{
		$task = new Execution();
		// Execute the following expression
		// (1+2)*(3+5)
		$task->evaluate("12+35+*");
		// -((7*2) + (6/2) + 4)
		$task->evaluate("72*62/+4+-");
	}
}
Execution::main(array());

Output

12+35+* = 24
72*62/+4+- = -21