Enabled Caching of Access Tokens

- Updated AuthTokenCredential to store access token in file storage
- Updated samples to include the configuration to disable/enable caching

sample/sdk_config.ini

@@ -1,13 +1,11 @@
+;## This is an example configuration file for the SDK.
+;## The sample scripts configure the SDK dynamically
+;## but you can choose to go for file based configuration
+;## in simpler apps (See bootstrap.php for more).
 [Account]
 acct1.ClientId = AYSq3RDGsmBLJE-otTkBtM-jBRd1TCQwFf9RGfwddNXWz0uFU9ztymylOhRS
 acct1.ClientSecret = EGnHDxD_qRPdaLdZz8iCr8N7_MzF-YHPTkjs6NKYQvQSBngp4PTTVWkPZRbL
 
-## This is an example configuration file for the SDK.
-## The sample scripts configure the SDK dynamically
-## but you can choose to go for file based configuration 
-## in simpler apps (See bootstrap.php for more).
-
-
 ;Connection Information
 [Http]
 http.ConnectionTimeOut = 30
@@ -16,11 +14,11 @@ http.Retry = 1
 
 ;Service Configuration
 [Service]
-mode=sandbox ; can be set to sandbox / live 
+; can be set to sandbox / live
+mode = sandbox
 
 ;Logging Information
 [Log]
-
 log.LogEnabled=true
 
 ; When using a relative path, the log file is created
@@ -42,4 +40,16 @@ log.LogLevel=FINE
 ; 'log'     : logs the error message to logger only (default)
 ; 'strict'  : throws a php notice message
 ; 'disable' : disable the validation
-validation.level=strict
+validation.level=log
+
+;Caching Configuration
+[cache]
+; If Cache is enabled, it stores the access token retrieved from ClientId and Secret from the
+; server into a file provided in constant $CACHE_PATH in PayPal/Cache/AuthorizationCache.
+; If the value is set to 'true', it would try to create a file and store the information.
+; For any other value, it would disable it
+; Please note, this is a very good performance improvement, and we would encourage you to
+; set this up properly to reduce the number of calls, to almost 50% on normal use cases
+; PLEASE NOTE: You may need to provide proper write permissions to /var directory under PayPal-PHP-SDK on
+; your hosting server
+cache.enabled=true