Trying to solve:

Strict warning: Declaration of CRM_Core_Payment_Stripe::singleton() should be
compatible with & CRM_Core_Payment::singleton().

And:
Strict warning: Accessing static property CRM_Core_Payment_Stripe::$_mode as
non static in CRM_Core_Payment_Stripe->__construct().

CRM/Core/Payment/Stripe.php

@@ -14,7 +14,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
   static private $_singleton = null;
 
   /**
-   * mode of operation: live or test
+   * Mode of operation: live or test.
    *
    * @var object
    * @static
@@ -30,7 +30,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
    * @return void
    */
   function __construct($mode, &$paymentProcessor) {
-    $this->_mode             = $mode;
+    self::$_mode             = $mode;
     $this->_paymentProcessor = $paymentProcessor;
     $this->_processorName    = ts('Stripe');
   }
@@ -38,17 +38,18 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
   /**
    * Singleton function used to manage this object.
    *
-   * @param string $mode
-   *   The mode of operation: live or test.
+   * @param string  $mode the mode of operation: live or test
+   * @param object  $paymentProcessor the details of the payment processor being invoked
+   * @param object  $paymentForm      reference to the form object if available
+   * @param boolean $force            should we force a reload of this payment object
    *
    * @return object
-   * 
    * @static
    *
    */
-  static function &singleton($mode, &$paymentProcessor) {
+  static function &singleton($mode = 'test', &$paymentProcessor, &$paymentForm = NULL, $force = FALSE) {
       $processorName = $paymentProcessor['name'];
-      if (self::$_singleton[$processorName] === NULL ) {
+      if (self::$_singleton[$processorName] === NULL || $force) {
           self::$_singleton[$processorName] = new self($mode, $paymentProcessor);
       }
       return self::$_singleton[$processorName];
@@ -59,7 +60,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
    *
    * @return string
    *   The error message if any.
-   * 
+   *
    * @public
    */
   function checkConfig() {
@@ -84,7 +85,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
 
   /**
    * Helper log function.
-   * 
+   *
    * @param string $op
    *   The Stripe operation being performed.
    * @param Exception $exception
@@ -97,12 +98,12 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
 
   /**
    * Run Stripe calls through this to catch exceptions gracefully.
-   * 
+   *
    * @param string $op
    *   Determine which operation to perform.
    * @param array $params
    *   Parameters to run Stripe calls on.
-   * 
+   *
    * @return varies
    *   Response from gateway.
    */
@@ -219,7 +220,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
    *
    * @return array
    *   The result in a nice formatted array (or an error object).
-   * 
+   *
    * @public
    */
   function doDirectPayment(&$params) {
@@ -444,9 +445,9 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
    * @param object $stripe_customer
    *   Stripe customer object generated by Stripe API.
    *
-   * @return array 
+   * @return array
    *   The result in a nice formatted array (or an error object).
-   * 
+   *
    * @public
    */
   function doRecurPayment(&$params, $amount, $stripe_customer) {
@@ -583,7 +584,7 @@ class CRM_Core_Payment_Stripe extends CRM_Core_Payment {
    *   Name value pair of contribution data.
    *
    * @return void
-   * 
+   *
    * @access public
    *
    */