diff --git a/library/log4php/LoggerNDC.php b/library/log4php/LoggerNDC.php
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+<?php
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * @package log4php
+ */
+
+/**
+ * The NDC class implements <i>nested diagnostic contexts</i>.
+ *
+ * NDC was defined by Neil Harrison in the article "Patterns for Logging
+ * Diagnostic Messages" part of the book <i>"Pattern Languages of
+ * Program Design 3"</i> edited by Martin et al.
+ *
+ * A Nested Diagnostic Context, or NDC in short, is an instrument
+ * to distinguish interleaved log output from different sources. Log
+ * output is typically interleaved when a server handles multiple
+ * clients near-simultaneously.
+ *
+ * This class is similar to the {@link LoggerMDC} class except that it is
+ * based on a stack instead of a map.
+ *
+ * Interleaved log output can still be meaningful if each log entry
+ * from different contexts had a distinctive stamp. This is where NDCs
+ * come into play.
+ *
+ * <b>Note that NDCs are managed on a per thread basis</b>.
+ *
+ * NDC operations such as {@link push()}, {@link pop()},
+ * {@link clear()}, {@link getDepth()} and {@link setMaxDepth()}
+ * affect the NDC of the <i>current</i> thread only. NDCs of other
+ * threads remain unaffected.
+ *
+ * For example, a servlet can build a per client request NDC
+ * consisting the clients host name and other information contained in
+ * the the request. <i>Cookies</i> are another source of distinctive
+ * information. To build an NDC one uses the {@link push()}
+ * operation.
+ *
+ * Simply put,
+ *
+ * - Contexts can be nested.
+ * - When entering a context, call <kbd>LoggerNDC::push()</kbd>
+ *     As a side effect, if there is no nested diagnostic context for the
+ *     current thread, this method will create it.
+ * - When leaving a context, call <kbd>LoggerNDC::pop()</kbd>
+ * - <b>When exiting a thread make sure to call {@link remove()}</b>
+ *
+ * There is no penalty for forgetting to match each
+ * <kbd>push</kbd> operation with a corresponding <kbd>pop</kbd>,
+ * except the obvious mismatch between the real application context
+ * and the context set in the NDC.
+ *
+ * If configured to do so, {@link LoggerPatternLayout} and {@link LoggerLayoutTTCC}
+ * instances automatically retrieve the nested diagnostic
+ * context for the current thread without any user intervention.
+ * Hence, even if a servlet is serving multiple clients
+ * simultaneously, the logs emanating from the same code (belonging to
+ * the same category) can still be distinguished because each client
+ * request will have a different NDC tag.
+ *
+ * Example:
+ *
+ * {@example ../../examples/php/ndc.php 19}<br>
+ *
+ * With the properties file:
+ *
+ * {@example ../../examples/resources/ndc.properties 18}<br>
+ *
+ * Will result in the following (notice the conn and client ids):
+ *
+ * <pre>
+ * 2009-09-13 19:04:27 DEBUG root conn=1234: just received a new connection in src/examples/php/ndc.php at 23
+ * 2009-09-13 19:04:27 DEBUG root conn=1234 client=ab23: some more messages that can in src/examples/php/ndc.php at 25
+ * 2009-09-13 19:04:27 DEBUG root conn=1234 client=ab23: now related to a client in src/examples/php/ndc.php at 26
+ * 2009-09-13 19:04:27 DEBUG root : back and waiting for new connections in src/examples/php/ndc.php at 29
+ * </pre>
+ *
+ * @version $Revision: 1350602 $
+ * @package log4php
+ * @since 0.3
+ */
+class LoggerNDC {
+
+    /** This is the repository of NDC stack */
+    private static $stack = array();
+
+    /**
+     * Clear any nested diagnostic information if any. This method is
+     * useful in cases where the same thread can be potentially used
+     * over and over in different unrelated contexts.
+     *
+     * <p>This method is equivalent to calling the {@link setMaxDepth()}
+     * method with a zero <var>maxDepth</var> argument.
+     */
+    public static function clear() {
+        self::$stack = array();
+    }
+
+    /**
+     * Never use this method directly, use the {@link LoggerLoggingEvent::getNDC()} method instead.
+     * @return array
+     */
+    public static function get() {
+        return implode(' ', self::$stack);
+    }
+
+    /**
+     * Get the current nesting depth of this diagnostic context.
+     *
+     * @see setMaxDepth()
+     * @return integer
+     */
+    public static function getDepth() {
+        return count(self::$stack);
+    }
+
+    /**
+     * Clients should call this method before leaving a diagnostic
+     * context.
+     *
+     * <p>The returned value is the value that was pushed last. If no
+     * context is available, then the empty string "" is returned.</p>
+     *
+     * @return string The innermost diagnostic context.
+     */
+    public static function pop() {
+        if (count(self::$stack) > 0) {
+            return array_pop(self::$stack);
+        } else {
+            return '';
+        }
+    }
+
+    /**
+     * Looks at the last diagnostic context at the top of this NDC
+     * without removing it.
+     *
+     * <p>The returned value is the value that was pushed last. If no
+     * context is available, then the empty string "" is returned.</p>
+     * @return string The innermost diagnostic context.
+     */
+    public static function peek() {
+        if (count(self::$stack) > 0) {
+            return end(self::$stack);
+        } else {
+            return '';
+        }
+    }
+
+    /**
+     * Push new diagnostic context information for the current thread.
+     *
+     * <p>The contents of the <var>message</var> parameter is
+     * determined solely by the client.
+     *
+     * @param string $message The new diagnostic context information.
+     */
+    public static function push($message) {
+        array_push(self::$stack, (string)$message);
+    }
+
+    /**
+     * Remove the diagnostic context for this thread.
+     */
+    public static function remove() {
+        LoggerNDC::clear();
+    }
+
+    /**
+     * Set maximum depth of this diagnostic context. If the current
+     * depth is smaller or equal to <var>maxDepth</var>, then no
+     * action is taken.
+     *
+     * <p>This method is a convenient alternative to multiple
+     * {@link pop()} calls. Moreover, it is often the case that at
+     * the end of complex call sequences, the depth of the NDC is
+     * unpredictable. The {@link setMaxDepth()} method circumvents
+     * this problem.
+     *
+     * @param integer $maxDepth
+     * @see getDepth()
+     */
+    public static function setMaxDepth($maxDepth) {
+        $maxDepth = (int)$maxDepth;
+        if (LoggerNDC::getDepth() > $maxDepth) {
+            self::$stack = array_slice(self::$stack, 0, $maxDepth);
+        }
+    }
+}