adds truncated signature example for PHP (#131)

* adds truncated signature example for PHP

* split truncated signature in a separate file

examples/signature-truncated.php

@@ -0,0 +1,33 @@
+<?php
+
+define('IMGPROXY_SIGNATURE_SIZE', 8 );
+
+$key = '943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881';
+$salt = '520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5';
+
+$keyBin = pack("H*" , $key);
+if(empty($keyBin)) {
+	die('Key expected to be hex-encoded string');
+}
+
+$saltBin = pack("H*" , $salt);
+if(empty($saltBin)) {
+	die('Salt expected to be hex-encoded string');
+}
+
+$resize = 'fill';
+$width = 300;
+$height = 300;
+$gravity = 'no';
+$enlarge = 1;
+$extension = 'png';
+
+$url = 'http://img.example.com/pretty/image.jpg';
+$encodedUrl = rtrim(strtr(base64_encode($url), '+/', '-_'), '=');
+
+$path = "/{$resize}/{$width}/{$height}/{$gravity}/{$enlarge}/{$encodedUrl}.{$extension}";
+$signature = hash_hmac('sha256', $saltBin.$path, $keyBin, true);
+$signature = pack('A'.IMGPROXY_SIGNATURE_SIZE, $signature);
+$signature = rtrim(strtr(base64_encode($signature), '+/', '-_'), '=');
+
+print(sprintf("/%s%s", $signature, $path));