Fixed minor bug introduced in previous commit.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4php/trunk@1382244 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/php/appenders/LoggerAppenderFile.php b/src/main/php/appenders/LoggerAppenderFile.php
index c9aad10..5d721ee 100644
--- a/src/main/php/appenders/LoggerAppenderFile.php
+++ b/src/main/php/appenders/LoggerAppenderFile.php
@@ -164,7 +164,7 @@
 		if (is_resource($this->fp)) {
 			$this->write($this->layout->getFooter());
 			fclose($this->fp);
-			unset($this->fp);
+			$this->fp = null;
 		}
 		$this->closed = true;
 	}