Fixed return values types in session drivers
diff --git a/system/libraries/Session/drivers/Session_files_driver.php b/system/libraries/Session/drivers/Session_files_driver.php
index d3ef34a..217d3f9 100644
--- a/system/libraries/Session/drivers/Session_files_driver.php
+++ b/system/libraries/Session/drivers/Session_files_driver.php
@@ -156,13 +156,13 @@
 				if (($this->_file_handle = fopen($this->_file_path.$session_id, 'w+b')) === FALSE)
 				{
 					log_message('error', "Session: File '".$this->_file_path.$session_id."' doesn't exist and cannot be created.");
-					return FALSE;
+					return '';
 				}
 			}
 			elseif (($this->_file_handle = fopen($this->_file_path.$session_id, 'r+b')) === FALSE)
 			{
 				log_message('error', "Session: Unable to open file '".$this->_file_path.$session_id."'.");
-				return FALSE;
+				return '';
 			}
 
 			if (flock($this->_file_handle, LOCK_EX) === FALSE)
@@ -170,7 +170,7 @@
 				log_message('error', "Session: Unable to obtain lock for file '".$this->_file_path.$session_id."'.");
 				fclose($this->_file_handle);
 				$this->_file_handle = NULL;
-				return FALSE;
+				return '';
 			}
 
 			// Needed by write() to detect session_regenerate_id() calls
diff --git a/system/libraries/Session/drivers/Session_memcached_driver.php b/system/libraries/Session/drivers/Session_memcached_driver.php
index 600b8ca..80d8b3d 100644
--- a/system/libraries/Session/drivers/Session_memcached_driver.php
+++ b/system/libraries/Session/drivers/Session_memcached_driver.php
@@ -170,7 +170,7 @@
 			return $session_data;
 		}
 
-		return FALSE;
+		return '';;
 	}
 
 	// ------------------------------------------------------------------------
diff --git a/system/libraries/Session/drivers/Session_redis_driver.php b/system/libraries/Session/drivers/Session_redis_driver.php
index c3c75b3..af427d2 100644
--- a/system/libraries/Session/drivers/Session_redis_driver.php
+++ b/system/libraries/Session/drivers/Session_redis_driver.php
@@ -171,7 +171,7 @@
 			return $session_data;
 		}
 
-		return FALSE;
+		return '';
 	}
 
 	// ------------------------------------------------------------------------