Update SIM.markdown

Add PHP flag for syntax highlighting

doc/SIM.markdown

@@ -18,7 +18,7 @@ reference.
 
 The code below will generate a buy now button that leads to a hosted order page:
 
-```
+```PHP
 <form method="post" action="https://test.authorize.net/gateway/transact.dll">
 <?php
 $amount = "9.99";
@@ -46,8 +46,9 @@ Fingerprint Generation
 
 To generate the fingerprint needed for a SIM transaction call the getFingerprint method:
 
-    $fingerprint = AuthorizeNetSIM_Form::getFingerprint($api_login_id, $transaction_key, $amount, $fp_sequence, $fp_timestamp);
-
+```PHP
+$fingerprint = AuthorizeNetSIM_Form::getFingerprint($api_login_id, $transaction_key, $amount, $fp_sequence, $fp_timestamp);
+```
 
 Relay Response
 --------------
@@ -65,7 +66,7 @@ http://yourdomain.com/response_handler.php and execute any logic you want
 when a transaction occurs. The AuthorizeNetSIM class makes it easy to verify
 the transaction came from Authorize.Net and parse the response:
 
-```
+```PHP
 $response = new AuthorizeNetSIM;
 if ($response->isAuthorizeNet())
 {