Some standard debugging functions I use in almost every project. Include this file after the main config file has been loaded.

php_debugging_functions.php

<?php
// This should probably be ported to a custom class...

// DEBUGGING - set to TRUE to force debugging or comment out to use dynamic
// debugging. The debugging behaviour is unreliable if this is set to FALSE
// define("X_GLOBAL_DEBUG", TRUE);

// LOG_DIR needs to be defined here rather than in the local config file because we
// may need access to it from pages where the local conf hasn't been loaded yet.
define('LOG_DIR', dirname(__FILE__) . '/../../logs');

// Define debugging levels. Set constant X_LOGVAR_LEVEL to one of these integer 
// values in the main config file.
define('X_LOGVAR_NONE', 0);
define('X_LOGVAR_ERROR', 1);
define('X_LOGVAR_WARNING', 2);
define('X_LOGVAR_INFO', 3);

// Override error logging defaults
ini_set('error_log', LOG_DIR . '/php_error.log');
ini_set('log_errors', 'On');
ini_set('html_errors', 'Off');
ini_set('display_errors', 'Off');

// Setup stricter error reporting if necessary
if ((defined('X_GLOBAL_DEBUG') && X_GLOBAL_DEBUG == true) //Forced debugging via constant
 || (isset($_REQUEST['X_GLOBAL_DEBUG']) && $_REQUEST['X_GLOBAL_DEBUG'] == 'foo') //Dynamic debugging via request parameter or cookie
) {
  error_reporting(E_ALL);
  if (!defined('X_GLOBAL_DEBUG')) {
    define('X_GLOBAL_DEBUG', true);
  }
} else {
  error_reporting(E_ALL ^ E_NOTICE);
  if (!defined('X_GLOBAL_DEBUG')) {
    define('X_GLOBAL_DEBUG', false);
  }
}

// Custom error reporting with backtrace
function log_error_backtrace($errno, $errstr, $errfile, $errline, $errcontext)
{
  if (!(error_reporting() & $errno)) return;
  switch ($errno) {
  case E_WARNING:
  case E_USER_WARNING:
  case E_STRICT:
  case E_NOTICE:
  case E_USER_NOTICE:
    $type = 'warning';
    $fatal = false;
    break;

  default:
    $type = 'fatal error';
    $fatal = true;
    break;
  }
  $log = array();
  foreach (array_reverse(debug_backtrace()) as $item) {
    $log[] = (isset($item['file']) ? $item['file'] : '<unknown file>') . ' ' . (isset($item['line']) ? $item['line'] : '<unknown line>') . ' calling ' . $item['function'] . '()';
  }
  LOGVAR($log, 'Backtrace from ' . $type . ' \'' . $errstr . '\' at ' . $errfile . ' ' . $errline, X_LOGVAR_ERROR);
  if (X_GLOBAL_DEBUG || ini_get('display_errors')) {
    PRINTVAR($log, 'Backtrace from ' . $type . ' \'' . $errstr . '\' at ' . $errfile . ' ' . $errline, (php_sapi_name() == 'cli'));
  }
  if ($fatal) exit(1);
}
// Comment out next line to disable custom error handler
set_error_handler('log_error_backtrace');

// Set X_GLOBAL_DEBUG_PLAIN to true if we are in a command-line-only environment
if (!isset($X_GLOBAL_DEBUG_PLAIN)) {
  $X_GLOBAL_DEBUG_PLAIN = false;
}

// The functions
function GETVAR($var, $label = "")
{
  $output = "<pre style=\"border: 1px solid #999; background-color: #f7f7f7; color: #000; overflow: auto; width: auto; text-align: left; padding: 1em;\">" . ((strlen(trim($label))) ? htmlentities($label) . "\n===================\n" : "") . htmlentities(print_r($var, true)) . "</pre>";
  return $output;
}
function GETVARPLAIN($var, $label = "")
{
  $output = '' . ((strlen(trim($label))) ? $label . "\n===================\n" : "") . print_r($var, true) . "\n\n";
  return $output;
}
function PRINTVAR($var, $label = "", $plain = false)
{
  global $X_GLOBAL_DEBUG_PLAIN;
  print (($plain || $X_GLOBAL_DEBUG_PLAIN) ? GETVARPLAIN($var, $label) : GETVAR($var, $label));
}
function PRINTVARD($var, $label = "", $plain = false)
{
  global $X_GLOBAL_DEBUG_PLAIN;
  die(($plain || $X_GLOBAL_DEBUG_PLAIN) ? GETVARPLAIN($var, $label) : GETVAR($var, $label));
}
function LOGVAR($var, $label = "", $level = X_LOGVAR_INFO, $group = false)
{
  if (!defined('X_LOGVAR_LEVEL')) {
    if (X_GLOBAL_DEBUG) {
      $X_LOGVAR_LEVEL = X_LOGVAR_INFO;
    } else {
      $X_LOGVAR_LEVEL = X_LOGVAR_WARNING;
    }
  } else {
    $X_LOGVAR_LEVEL = X_LOGVAR_LEVEL;
  }
  if (X_GLOBAL_DEBUG) {
    PRINTVAR($var, $label);
  } elseif (intval($level) > $X_LOGVAR_LEVEL) {
    return;
  } else {
    $message = GETVARPLAIN($var, $label);
    if (X_LOGVAR_USE_DATABASE) {
      debug_log_to_database($message, $level, ($group) ? $group : 'debug');
    }
    else {
      $uri = $_SERVER['REQUEST_URI'];
      if (!empty($_SERVER['QUERY_STRING'])) $uri.= '?' . $_SERVER['QUERY_STRING'];
      $data = sprintf("[%s] Request URI: %s\n-------------------------------------------------\n%s", gmdate('d-M-Y H:i:s') . ' GMT', $uri, $message);
      debug_write_log(($group) ? $group : 'debug', $data);
    }
  }
}
function LOGVARD($var, $label = "", $group = false)
{
  if (X_GLOBAL_DEBUG) {
    PRINTVARD($var, $label);
  } else {
    LOGVAR($var, $label, 99, $group);
    PRINTVARD('A fatal error has occured and has been logged.');
  }
}
function debug_write_log($logfile, $data)
{
  $filename = sprintf("%s/%s.%s.%s.log", LOG_DIR, $logfile, date('Y-m-d'), str_replace('.', '_', $_SERVER['SERVER_ADDR']));
  $fp = @fopen($filename, 'a+b');
  if ($fp === false || !is_writeable($filename)) {
    PRINTVARD('Could not open log file for writing');
  }
  fwrite($fp, $data);
  fclose($fp);  
}

/**
 * To enable tracking of data across requests, add the following to your config file:
 *
   // Though not neccessarily a configurable option, this is the best place to include this
   // so that we have access to the COOKIE_HOST constant.
   if (empty($_COOKIE['x_logvar_id']) && !headers_sent()) {
     $x_logvar_id = base_convert(mt_rand(0, 0x38E38E3), 10, 36) . '-' . microtime(TRUE);
     setcookie("x_logvar_id", $x_logvar_id, 0, '/', '' . COOKIE_HOST, FALSE);
     setcookie("x_logvar_id", $x_logvar_id, 0, '/', '.' . COOKIE_HOST, FALSE);
   }
 */
function debug_log_to_database($message, $level, $group) {
  $log_data = array(
    'message'   => $message,
    'level'     => $level,
    'group'     => $group,
    'timestamp' => gmdate('Y-m-d H:i:s'),
    'request_environment' => GETVARPLAIN(array(
      'SERVER'  => $_SERVER,
      'GET'     => $_GET,
      'POST'    => $_POST,
      'FILES'   => $_FILES,
      'COOKIE'  => $_COOKIE,
      'SESSION' => $_SESSION,
      'REQUEST' => $_REQUEST,
      'ENV'     => $_ENV
    )),
    'remote_address' => ((!empty($_SERVER['REMOTE_ADDR'])) ? $_SERVER['REMOTE_ADDR'] : ''),
    'uri'            => ((!empty($_SERVER['REQUEST_URI'])) ? $_SERVER['REQUEST_URI'] : ''),
    'server_address' => ((!empty($_SERVER['SERVER_ADDR'])) ? $_SERVER['SERVER_ADDR'] : ''),
    'x_logvar_id'    => ((!empty($_COOKIE['x_logvar_id'])) ? $_COOKIE['x_logvar_id'] : '')
  );
  if (function_exists('some_insert_function')) {
    /**
     * Add code to insert the $log_data array into the database. If we were using ADOdb, it might
     * look like this:
     *
     global $ADODB_QUOTE_FIELDNAMES;
     $old_ADODB_QUOTE_FIELDNAMES = $ADODB_QUOTE_FIELDNAMES;
     // Make sure ADOdb quotes the field names
     $ADODB_QUOTE_FIELDNAMES = TRUE;
     some_insert_function('_debug_log', $log_data);
     $ADODB_QUOTE_FIELDNAMES = $old_ADODB_QUOTE_FIELDNAMES;
     unset($old_ADODB_QUOTE_FIELDNAMES);
    */
   }
  // If all else fails:
  else {
    debug_write_log($group, $log_data);
  }
}
$X_GLOBAL_BENCHMARK_LAST_TIME = $X_GLOBAL_BENCHMARK_TIME = microtime(true);
function benchmark($label = false, $save_to_log = false, $group = false)
{
  if (!$label) {
    $trace = debug_backtrace();
    if (isset($trace[1]))
    {
      $label = (empty($trace[1]['function'])) ? '' : "{$trace[1]['function']}::";
      $label .= "{$trace[0]['file']}::{$trace[0]['line']}";
    }
  }
  $btime = microtime(true);
  LOGVAR(array(
    'benchmark' => $btime - $GLOBALS['X_GLOBAL_BENCHMARK_TIME'],
    'delta'     => $btime - $GLOBALS['X_GLOBAL_BENCHMARK_LAST_TIME']
  ), $label, X_LOGVAR_INFO, $group);
  if ($save_to_log) {
    debug_write_log(($group) ? $group : 'benchmark', sprintf("[%s]\t%s\t%s\n", $label, gmdate('d-M-Y H:i:s') , $btime - $GLOBALS['X_GLOBAL_BENCHMARK_TIME']));
  }
  $GLOBALS['X_GLOBAL_BENCHMARK_LAST_TIME'] = $btime;
}
?>