🧩 Caesar Cipher Decoder (PHP)

This project is a simple Caesar Cipher Decoder written in PHP.
It takes an encoded text and a numeric shift value, and returns the decoded message.

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🧠 What is a Caesar Cipher?

A Caesar cipher is a basic encryption technique in which each letter in a message is replaced
by another letter a fixed number of positions down the alphabet.

Example:

Plain text:  HELLO
Shift:       3
Encoded:     KHOOR

Decoding reverses this shift to recover the original message.

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⚙️ Requirements

• PHP 7.0+ installed on your system (macOS already includes PHP by default).
  You can check your version by running:

  php -v

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📦 Installation

1. Clone or download this project to your computer.

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▶️ Usage

Option 1 — Run with custom arguments

You can pass the encoded text and shift value directly from the terminal.

php decoder.php "Mjqqt Btwqi" 5

Output:

Decoded text: Hello World

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Option 2 — Run with no arguments

If no arguments are provided, the script runs a built-in example automatically:

php decoder.php

Output:

No arguments provided. Running example...

Encoded text: Aopz pz aol huzdly, dlss kvul!
Shift: 7

Decoded text: This is the answer, well done!

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💡 How It Works

• Each alphabetic character is shifted back by the given $shift value.
• Uppercase and lowercase letters are preserved.
• Non-alphabetic characters (spaces, punctuation, etc.) are not changed.
• The alphabet “wraps around” — shifting 'A' back by 1 becomes 'Z'.

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🧩 Example Test Cases

Encoded Text	Shift	Expected Output
Czggj Rjmgy	5	Hello World
Dpef! Xibu jt uijt?	1	Code! What is this?
Olssv Dvysk	33	Hello World
Aopz pz aol huzdly, dlss kvul!	7	This is the answer, well done!

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🧾 License

This project is open-source and free to use for educational or professional purposes.

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👨‍💻 Author: Thiago Vinhas
📅 Version: 1.0