Added SPL implementation of a Stack example

Author: Alienpruts

chap-04/SplStack.php

@@ -0,0 +1,18 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 16:40
+ */
+
+// SPL implementation of the Stack uses the concept of Doubly Linked Lists.
+
+$books = new SplStack();
+
+$books->push("Introduction to PHP7");
+$books->push("Mastering Javascript");
+$books->push("MySQL Workbench tutorial");
+
+echo $books->pop() . PHP_EOL;
+echo $books->top() . PHP_EOL;