A simple PHP logger for ease of use with the decorator design pattern. This contains the abstract base class which should be extended in the least implementing _BaseLogger::resolveMessage(). As an example, this includes a sample stream logger.

ExpandableClass.php

<?php
/*******************************************************************************
* Expandable Class
*
* Authors:: anthony.gallagher@wellspringworldwide.com
*
* Copyright:: Copyright 2009, Wellspring Worldwide, LLC Inc. All Rights Reserved.
* Licensed 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.
* Source: https://github.com/wellspringworldwide/PHP-ClassMixer/blob/master/Mixins.php
******************************************************************************/
/*******************************************************************************
* Enables the dynamic addition of new methods or expander classes with
* extra functionality after the class has been defined.
******************************************************************************/
class ExpandableClassException extends Exception {}

abstract class ExpandableClassMixin {
    static protected $dynamic_methods = array();
    static protected $dynamic_mixins = array();

    /**
* Helper method to generate an argument string to eval()
*
* @param <type> $num_args
* @return <type>
*/
    protected static function makeArgStr($num_args) {
        $m_args = array();
        for ($i = 0; $i < $num_args; $i++) {
            $m_args[] = "\$args[$i]";
        }
        return implode(', ', $m_args);
    }

    protected static function getClassHierarchy($base_object) {
        $ancestry = array();
        $klass = is_object($base_object) ? get_class($base_object) : $base_object;
        $ancestry[] = $klass;
        while (($klass = get_parent_class($klass)) !== false) {
            $ancestry[] = $klass;
        }
        return $ancestry;
    }

    /***************************************************************************
* Dynamic method registration
**************************************************************************/
    /**
* Check if a method has been dynamically added to a class
* @param <type> $klass
* @param <type> $method
* @return <type>
*/
    protected static function isMethodRegistered($klass, $method) {
        if (!array_key_exists($klass, self::$dynamic_methods)) {
            self::$dynamic_methods[$klass] = array();
        }
        return isset(self::$dynamic_methods[$klass][$method]);
    }

    /**
* Dynamically add a method from a class. Methods should be functions with
* a signature of:
* function NEW_ClassName__MethodName($obj, ...) { ... }
* where $obj is the receiver for the $this pointer.
* @param <type> $klass
* @param <type> $method
*/
    public static function registerMethod($klass, $method) {
        //The name of the function to be called
        if (!self::isMethodRegistered($klass, $method)) {
            $dynamic_method_name = "NEW_".$klass."__".$method;
            self::$dynamic_methods[$klass][$method] = $dynamic_method_name;
        }
    }

    /**
* Dynamically remove a method from a class
* @param <type> $klass
* @param <type> $method
*/
    public static function unregisterMethod($klass, $method) {
        if (self::isMethodRegistered($klass, $method)) {
            unset(self::$dynamic_methods[$klass][$method]);
        }
    }

    /***************************************************************************
* Dynamic mixin registration
**************************************************************************/
    /**
* Get the expanders for a given class
*
* @param <type> $klass
* @return <type>
*/
    public static function getExpanders($klass) {
        if (!array_key_exists($klass, self::$dynamic_mixins)) {
            self::$dynamic_mixins[$klass] = array();
        }
        return self::$dynamic_mixins[$klass];
    }

    /**
* Check if an expander mixin has been dynamically added to a class
*
* @param <type> $klass
* @param <type> $expander
* @return <type>
*/
    public static function isExpanderRegistered($klass, $expander) {
        return in_array($expander, self::getExpanders($klass));
    }

    /**
* Dynamically add a mixin to a class.
*
* @param <type> $klass
* @param <type> $expander
*/
    public static function registerExpander($klass, $expander) {
        //The name of the function to be called
        if (!self::isExpanderRegistered($klass, $expander)) {
            self::$dynamic_mixins[$klass][] = $expander;
        }
    }

    /**
* Dynamically remove a mixin from a class
*
* @param <type> $klass
* @param <type> $expander
*/
    public static function unregisterExpanders($klass, $expander) {
        if (self::isExpanderRegistered($klass, $expander)) {
            self::$dynamic_mixins[$klass] =
                array_values(array_diff(self::$dynamic_mixins[$klass], array($expander)));
        }
    }

    /**
* Get all dynamically added methods
*
* @param <type> $object_or_klass
* @param <type> $method
*/
    public static function get_dynamic_class_methods($object_or_klass) {
        $methods = array();
        $klasses = self::getClassHierarchy($object_or_klass);
        foreach ($klasses as $klass) {
            //Add the dynamically added methods
            $dyn_methods = isset(self::$dynamic_methods[$klass]) ? array_keys(self::$dynamic_methods[$klass]) : array();
            $methods = array_merge($methods, $dyn_methods);

            //Add the expander methods
            $expanders = self::getExpanders($klass);
            foreach ($expanders as $expander) {
                //Get the methods of the expander class
                $exp_methods = get_class_methods($expander);
                $methods = array_merge($methods, $exp_methods);
            }
        }

        return $methods;
    }

    /**
* Find where a dynamic method for a class is registered
*
* @param <type> $object_or_klass
* @param <type> $method
* @return <type>
*/
    public static function find_dynamic_class_method($object_or_klass, $method) {
        $klasses = self::getClassHierarchy($object_or_klass);
        foreach ($klasses as $klass) {
            //Return the dynamic method if found
            if (self::isMethodRegistered($klass, $method)) {
                return array(null, "NEW_".$klass."__".$method);
            }

            //Get the expanders for the class
            $expanders = self::getExpanders($klass);
            foreach ($expanders as $expander) {
                //Get the methods of the expander class
                $methods = get_class_methods($expander);
                //Found the method!
                if (in_array($method, $methods)) {
                    return array($expander, $method);
                }
            }
        }

        return null;
    }

    /**
* Checks if a class has the method dynamically added
*
* @param <type> $object_or_klass
* @param <type> $method
* @return <type>
*/
    public static function dynamic_method_exists($object_or_klass, $method) {
        return is_array(self::find_dynamic_class_method($object_or_klass, $method));
    }

    /**
* Here look for expander classes that may define the method called on the object
* When using this mixin, the combinator for this method should be rerouted to
* the resulting class's __call method
*
* @param <type> $method
* @param <type> $args
* @return <type>
*/
    public function ___call($method, $args) {
        //Get the list of classes we need to check for expander registration
        $klasses = self::getClassHierarchy($this);

        foreach ($klasses as $klass) {
            //Check if this is a dynamically added method, if so call the method
            if (self::isMethodRegistered($klass, $method)) {
                //A dynamically added method
                array_unshift($args, $this);
                $dynamic_method_name = "NEW_".$klass."__".$method;
                eval("\$result =& $dynamic_method_name(".self::makeArgStr(count($args)).");");
                return $result;
            }

            //Get the expanders for the class
            $expanders = self::getExpanders($klass);
            foreach ($expanders as $expander) {
                //Get the methods of the expander class
                $methods = get_class_methods($expander);
                if (is_null($methods) && !class_exists($expander)) {
                    throw new Exception("Expander for `$klass` - `$expander` - is not a valid class. Please check the expander registration.");
                }
                //Found the method! Call it and return the result!
                if (in_array($method, $methods)) {
                    eval("\$result =& $expander::$method(".self::makeArgStr(count($args)).");");
                    return $result;
                }
            }
        }

        //The method was not found...Trigger an exception.
        throw new ExpandableClassException('Call to undefined method '.get_class($this).'::'.$method);
    }
}

/**
* Just a pseudonym for the ExpandableClassMixin that we can inherit from
*/
abstract class ExpandableClass extends ExpandableClassMixin {
    public function __call($method, $args) {
        return parent::___call($method, $args);
    }
}

Logger.php

<?php
/**
 * Copyright 2011 Zack Douglas <zackery.douglas@gmail.com>
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * defines an interface and basic methods for implementing a decorator-pattern
 * logging framework.
 *
 * In this implementation, we are using the ExpandableClass as our base,
 * derived from
 *   https://github.com/wellspringworldwide/PHP-ClassMixer
 * but this may just as easily extend from stdClass.
 * 
 * As an example usage:
 * $logger = new FooLogger(E_ERROR);
 * $logger->persist_args = true;
 * $logger = new BarLogger(array('para1'=>'val1'), E_WARNING, $logger);
 * $logger = new BazLogger('some_argument', E_NOTICE, $logger);
 * // more decorations?!
 * $logger->log(E_WARNING, 'My Warning');
 * try {
 *     throw new ExampleError('My Error');
 * } catch (ExampleError $e) {
 *     // in this example all loggers in the decoration chain will fire, as
 *     // logError() defaults to E_ERROR unless specified as the second argument
 *     $logger->logError($e);
 * }
 * NOTICE that we use the E_* constants, not LOG_*; this is INTENTIONAL. As of
 * PHP 5.3.0, LOG_* is DEPRECATED, as well as being documented as part of the
 * Network service
 */
abstract class _BaseLogger extends ExpandableClass {
        /**
         * the integer log level. A level less than zero indicates logging all.
         */
    var $lvl = -1,
        /**
         * indicates that the logger should resolve its errors while executing,
         * rather than with an explicit call to resolve(),
         * or at time of destruction if $on_destruct is true
         */
        $synchronized = false,
        /**
         * indicates whether the logger should implicitly resolve() when
         * garbage collected
         */
        $resolve_on_destruct = true,
        /**
         * an optional wrapped logger which should be notified of all events when
         * this logger is triggered
         */
        $wrapped = null,
        /**
         * queue of messages received from event methods
         */
        $messages = array(),
        /**
         * persists certain arguments (synchronized, on_destruct) set on this
         * logger to all decorating loggers. Default is false.
         */
        $persist_args = false;

    /**
     * constructs a logger with minimal effort. You may optionally define both a
     * logging level and a wrapped logger. By default, new loggers will listen
     * to all messages; this is by design.
     * @param int $lvl the logging level this logger will record
     * @param _BaseLogger $wrapped the logger which we're decorating
     */
    public function __construct($lvl=-1, _BaseLogger $wrapped=null) {
        $this->lvl = $lvl;
        $this->wrapped = $wrapped;
        if ($wrapped->persist_args) {
            $this->persist_args = true;
            $this->synchronized = $wrapped->synchronized;
            $this->resolve_on_destruct = $wrapped->resolve_on_destruct;
        }
    }

    /**
     * is fired when the logger loses scope and is garbage collected. If
     * on_destruct is true, the logger chain will be resolve()'d
     */
    public function __destruct() {
        if ($this->resolve_on_destruct) {
            $this->resolve();
        }
    }

    /**
     * provides a public method which can be called from decorating loggers on
     * decorated loggers to store new messages
     * @param int $lvl the indicator level
     * @param str $msg the message to record
     */
    public function log($lvl, $msg) {
        $this->storeConditional($lvl, $msg);
        if ($this->wrapped) {
            $this->wrapped->log($lvl, $msg);
        }
        if ($this->synchronized) {
            $this->resolve();
        }
    }

    /**
     * is meant to be used in a try-catch context where an Exception is raised
     * but could be recovered from. This collects the message, file, and line
     * number from the Exception argument
     * @param Exception $e the exception to intercept and record
     * @param int $lvl an optional level override, defaulting to E_ERROR
     */
    public function logError(Exception $e, $lvl=E_ERROR) {
        $cls = get_class($e);
        $msg = "{$cls}: {$e->getMessage()} (Code {$e->getCode()})on line {$e->getLine()} of file {$e->getFile()}\nFULL TRACE:\n{$e->getTraceAsString()}";
        $this->log($lvl, $msg);
    }

    /**
     * determines whether to record the $msg based upon the internal logging
     * level and the provided level of the $msg
     * @param int $lvl the indicator level
     * @param str $msg the message to record
     */
    protected function storeConditional($lvl, $msg) {
        $lvl_text = $this->getLogLevelText($lvl);
        $dt = date('c');
        if ($this->lvl >= $lvl || $this->lvl < 0) {
            $this->messages []= "[ {$lvl_txt} ] {$dt} {$msg}";
        }
    }

    /**
     * provides a public method which can be called from decorating loggers to
     * the decorated loggers to resolve their messages using resolveMessage().
     * Messages are removed as they are individually resolved, allowing a
     * modicum of disaster recovery.
     */
    public function resolve() {
        $message_keys = array_keys($this->messages);
        foreach ($message_keys as $key) {
            $this->resolveMessage($this->messages[$key]);
            unset ($this->messages[$key]);
        }
        if ($this->wrapped) {
            $this->wrapped->resolve();
        }
    }

    /**
     * resolves an individual message for the current logger
     * @param str $msg the message to record
     */
    protected function resolveMessage($msg) {
        throw new NotImplementedException(__class__, __function__);
    }

    /**
     * nullifies all recorded messages in the logger chain
     */
    public function nullify() {
        $this->messages = array();
        if ($this->wrapper) {
            $this->wrapper->nullify();
        }
    }

    /**
     * from an integer log level, retrieve the corresponding human-readable text
     * @param int $lvl the indicator level
     */
    public function getLogLevelText($lvl) {
        if (E_ERROR === $lvl) {
            return 'ERROR';
        } elseif (E_WARNING === $lvl) {
            return 'WARNING';
        } elseif (E_PARSE === $lvl) {
            return 'PARSE';
        } elseif (E_NOTICE === $lvl) {
            return 'NOTICE';
        } elseif (E_STRICT === $lvl) {
            return 'STRICT';
        } else {
            return 'DEBUG';
        }
    }
}

StreamLogger.php

<?php
/**
 * Copyright 2011 Zack Douglas <zackery.douglas@gmail.com>
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
 
/**
 * records log messages to a file or stream, delimited (by default) by the
 * PHP_EOL constant.
 *
 * As an example usage:
 * $logger = new StreamLogger(fopen('php://stderr', 'w'), E_WARN);
 * // possible decorating loggers
 * try {
 *     $logger->log(E_WARNING, 'Example warning');
 *     throw new ExampleError('Example error');
 * } catch (ExampleError $e) {
 *     $logger->logError($e); // default level is E_ERROR
 * }
 */
class StreamLogger extends _BaseLogger {
        /**
         * handle to a stream for writing
         */
    var $stream = null,
        /**
         * whether this instance created a PHP stream
         */
        $stream_created = false,
        /**
         * default message delimiter to the stream
         */
        $delimiter = PHP_EOL;

    /**
     * from a location or stream, initialize parameters for logging to a stream
     * @param str|stream $location_or_stream if a string, a new stream will be
     * attempted to be opened for binary safe writing
     * @param int $lvl the logging level
     * @param _BaseLogger $wrapped an optional logger to decorate
     */
    public function __construct($location_or_stream, $lvl, _BaseLogger $wrapped=null) {
        if (is_string($location_or_stream)) {
            $this->stream = @fopen($location_or_stream, 'ab');
            if (!$this->stream) {
                throw new RuntimeException("Could not open stream to {$location_or_stream} for appending");
            }
            $this->stream_created = true;
        } else {
            $meta = stream_get_meta_data($location_or_stream);
            if (false === strpos($meta['mode'], 'a')
                ||
                false === strpos($meta['mode'], 'w')
            ) {
                throw new RuntimeException("Stream provided not available for writing");
            }
            $this->stream = $location_or_handle;
        }
        parent::__construct($lvl, $wrapped);
    }

    /**
     * cleans up freshly allocated streams on garbage collection
     */
    public function __destruct() {
        if ($this->stream_created) {
            fclose($this->stream);
        }
    }

    /**
     * records an individual message to the stream
     * @param str $msg the log message to resolve
     */
    protected function resolveMessage($msg) {
        fwrite($this->stream, "{$msg}{$this->delimiter}");
    }
}