Magento Checkout is broken after SUPEE-11085 with PHP<5.5

Magento checkout page is broken after installing SUPEE-11085 (Authorize.net Direct Post Signature Key patch). Proceed to Checkout button results in a blank page, PHP fatal error is thrown:

Fatal error: Can't use method return value in write context in app/code/core/Mage/Authorizenet/Model/Directpost.php on line 392 

Cause

empty() function used in the patch with a function return value. As per official PHP documentation:

Prior to PHP 5.5, empty() function only supports variables; anything else will result in a parse error. In other words, the following will not work: empty(trim($name)). Instead, use trim($name) == false.

Solution

Upgrade to newer PHP version (i.e. use our service to switch to faster PHP 7.2) or change the function usage according to your PHP version:

--- app/code/core/Mage/Authorizenet/Model/Directpost.php
+++ app/code/local/Mage/Authorizenet/Model/Directpost.php
@@ -389,7 +389,8 @@
     public function validateResponse()
     {
         $response = $this->getResponse();
-        $hashConfigKey = !empty($response->getData('x_SHA2_Hash')) ? 'signature_key' : 'trans_md5';
+       $response_getData_hash = $response->getData('x_SHA2_Hash');
+        $hashConfigKey = !empty($response_getData_hash) ? 'signature_key' : 'trans_md5';
 
         //hash check
         if (!$this->getConfigData($hashConfigKey)
--- app/code/core/Mage/Authorizenet/Model/Directpost/Request.php
+++ app/code/local/Mage/Authorizenet/Model/Directpost/Request.php
@@ -187,7 +187,8 @@
     {
         $fpTimestamp = time();
 
-        if (!empty($this->_getSignatureKey())) {
+       $this_getSignatureKey = $this->_getSignatureKey();
+        if (!empty($this_getSignatureKey)) {
             $hash = $this->_generateSha2RequestSign(
                 $this->getXLogin(),
                 $this->_getSignatureKey(),
--- app/code/core/Mage/Authorizenet/Model/Directpost/Response.php
+++ app/code/local/Mage/Authorizenet/Model/Directpost/Response.php
@@ -56,14 +56,17 @@
      */
     public function isValidHash($storedHash, $merchantApiLogin)
     {
-        if (empty($this->getData('x_amount'))) {
+       $this_getData_x_amount = $this->getData('x_amount');
+        if (empty($this_getData_x_amount)) {
             $this->setData('x_amount', '0.00');
         }
 
-        if (!empty($this->getData('x_SHA2_Hash'))) {
+       $this_getData_x_SHA2_Hash = $this->getData('x_SHA2_Hash');
+       $this_getData_x_MD5_Hash = $this->getData('x_MD5_Hash');
+        if (!empty($this_getData_x_SHA2_Hash)) {
             $hash = $this->generateSha2Hash($storedHash);
             return $hash == $this->getData('x_SHA2_Hash');
-        } elseif (!empty($this->getData('x_MD5_Hash'))) {
+        } elseif (!empty($this_getData_x_MD5_Hash)) {
             $hash = $this->generateHash($storedHash, $merchantApiLogin, $this->getXAmount(), $this->getXTransId());
             return $hash == $this->getData('x_MD5_Hash');
         }

Or use our ZIP bundle with these local overrides for PHP 5.4 and earlier versions from here:

Magento version	SUPEE-11085 ZIP bundle
CE 1.9.2.4 - CE 1.9.4.0 (PHP5.5 and newer)	SUPEE-11085-1.9.x
CE 1.9.2.4 - CE 1.9.4.0 (PHP5.4 and older)	SUPEE-11085-1.9.x-oldPHP

or use our patch installation service to install all missing patches at once.

If you have any difficulties with solving this problem or got a similar one, please let us know in comments below, so we can find the solution together.