Add PHP example based on DS extension for chapter 08 - greedy algorithms (#97)

* Add PHP example based on DS extension for chapter 08 - greedy algorithms

* Add code formatting

* Add code formatting / 2

08_greedy_algorithms/php/01_set_covering.php

@@ -0,0 +1,36 @@
+<?php
+// You need to install Data Structures (DS) extension. 
+// More details here - http://php.net/manual/en/book.ds.php
+use \Ds\Set;
+
+$statesNeeded = new Set(["mt", "wa", "or", "id", "nv", "ut", "ca", "az"]);
+$stations = [];
+$stations["kone"] = new Set(["id", "nv", "ut"]);
+$stations["ktwo"] = new Set(["wa", "id", "mt"]);
+$stations["kthree"] = new Set(["or", "nv", "ca"]);
+$stations["kfour"] = new Set(["nv", "ut"]);
+$stations["kfive"] = new Set(["ca", "az"]);
+$finalStations = new Set();
+
+while (!$statesNeeded->isEmpty()) {
+    $bestStation = null;
+    $statesCovered = new Set();
+    foreach (array_keys($stations) as $station) {
+        $states = $stations[$station];
+        $covered = new Set($statesNeeded);
+        $covered = $covered->filter(function ($value) use ($states) {
+            return $states->contains($value);
+        });
+        if ($covered->count() > $statesCovered->count()) {
+            $bestStation = $station;
+            $statesCovered = $covered;
+        }
+    }
+    $statesNeeded = new Set($statesNeeded);
+    $statesNeeded = $statesNeeded->filter(function ($value) use ($statesCovered) {
+        return !$statesCovered->contains($value);
+    });
+    $finalStations->add($bestStation);
+}
+
+print_r($finalStations); // ["kone", "ktwo", "kthree", "kfive"]