add free example

bindings/php/examples/free.php

@@ -0,0 +1,20 @@
+<?php
+$sigar = new Sigar();
+
+$mem = $sigar->mem();
+
+$swap = $sigar->swap();
+
+echo "\tTotal\tUsed\tFree\n";
+
+echo "Mem:    " .
+      ($mem->total() / 1024) . "\t" .
+      ($mem->used() / 1024) . "\t" .
+      ($mem->free() / 1024) . "\n";
+
+echo "Swap:   " .
+      ($swap->total() / 1024) . "\t" .
+      ($swap->used() / 1024) . "\t" .
+      ($swap->free() / 1024) . "\n";
+
+echo "RAM:    " . $mem->ram() . "MB\n";