Add XOR Cipher

DIRECTORY.md

@@ -1,7 +1,5 @@
-## String
-  * [Check Palindrome](https://github.com/TheAlgorithms/PHP/blob/master/String/CheckPalindrome.php)
-  * [Reverse String](https://github.com/TheAlgorithms/PHP/blob/master/String/ReverseString.php)
-  * [Reverse Words](https://github.com/TheAlgorithms/PHP/blob/master/String/ReverseWords.php)
+## Ciphers
+  * [Xor Cipher](https://github.com/TheAlgorithms/PHP/blob/master/ciphers/XORCipher.php)
   
 ## Searching
   * [Binary Search](https://github.com/TheAlgorithms/PHP/blob/master/searches/binary_search.php)
@@ -12,3 +10,8 @@
   * [Merge Sort](https://github.com/TheAlgorithms/PHP/blob/master/sorting/mergeSort.php)
   * [Radix Sort](https://github.com/TheAlgorithms/PHP/blob/master/sorting/radixSort.php)
   * [Selection Sort](https://github.com/TheAlgorithms/PHP/blob/master/sorting/selectionSort.php)
+
+## String
+  * [Check Palindrome](https://github.com/TheAlgorithms/PHP/blob/master/String/CheckPalindrome.php)
+  * [Reverse String](https://github.com/TheAlgorithms/PHP/blob/master/String/ReverseString.php)
+  * [Reverse Words](https://github.com/TheAlgorithms/PHP/blob/master/String/ReverseWords.php)

ciphers/XORCipher.php

@@ -0,0 +1,18 @@
+<?php
+// The XOR cipher is a type of additive cipher.
+// Each character is bitwise XORed with the key.
+// We loop through the input string, XORing each
+// character with the key.
+function xor_cipher(string $inp_string, string $key)
+{
+
+    $key_len = strlen($key);
+    $result = array();
+
+    for ($idx = 0;$idx < strlen($inp_string);$idx++)
+    {
+        array_push($result, $inp_string[$idx] ^ $key[$idx % $key_len]);
+    }
+
+    return join("", $result);
+}

tests/CipherTest.php

@@ -0,0 +1,22 @@
+<?php
+
+use function PHPUnit\Framework\assertEquals;
+use function PHPUnit\Framework\assertFalse;
+use function PHPUnit\Framework\assertNotEquals;
+use function PHPUnit\Framework\assertTrue;
+use PHPUnit\Framework\TestCase;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+require_once __DIR__ . '/../ciphers/XORCipher.php';
+
+class CipherTest extends TestCase
+{
+    public function testXor_cipher()
+    {
+        $inp_str = "test@string";
+        $key = "test-key";
+        $invalid_key = "wrong-key";
+        assertEquals( $inp_str, xor_cipher( xor_cipher( $inp_str , $key) , $key));
+        assertNotEquals( $inp_str, xor_cipher( xor_cipher( $inp_str , $key) , $invalid_key));
+    }
+}