Family Day update

Task/Entropy/PHP/entropy.php

@@ -0,0 +1,27 @@
+<?php
+
+function shannonEntropy($string) {
+    $h = 0.0;
+    $len = strlen($string);
+    foreach (count_chars($string, 1) as $count) {
+        $h -= (double) ($count / $len) * log((double) ($count / $len), 2);
+    }
+    return $h;
+}
+
+$strings = array(
+    '1223334444',
+    '1225554444',
+    'aaBBcccDDDD',
+    '122333444455555',
+    'Rosetta Code',
+    '1234567890abcdefghijklmnopqrstuvwxyz',
+);
+
+foreach ($strings AS $string) {
+    printf(
+        '%36s : %s' . PHP_EOL,
+        $string,
+        number_format(shannonEntropy($string), 6)
+    );
+}