This is my first solution to the RPN Calculator Kata proposed here using php and phpUnit.

A RPN calculator program computes expressions written in RPN (Reverse Polish Notation). A RPN expression (or a postfix expression) is one of the following:

• a number X, in which case the value of the expression is that of X;
• a sequence of the form E1 E2 O, where E1 and E2 are postfix expressions and O is an arithmetic operation; in this case, the value of the expression is that of E1 O E2

The following are RPN expressions:

• 20 5 / => (20/5) = 4
• 4 2 + 3 - => (4+2)-3 = 3
• 3 5 8 * 7 + * => 3*((5*8)+7) = 141
• 7 2 - 3 4 = 5 3 4

Suggested scenarios

• Given a RPN Calculator when a digit is sent it should display the same digit
• Given a RPN Calculator when some digits are sent it should display the number formed by those digits
• Given a RPN Calculator when an operation (* + / -) is sent after two numbers it should display the result of that operation