debug.php

<?php
class Debug {
	private static $log = array();
	
	public static function trigger($type, $data, &$caller) {
		if(!isset(self::$log[$type])) self::$log[$type] = array();
		if(!isset(self::$log[$type.'_func'])) self::$log[$type.'_func'] = array();
		self::$log[$type][] = $data;
		self::$log[$type.'_func'][] = array($caller[0]['file'],$caller[0]['line'],(!empty($caller[1]['class']) ? '<br/>'.$caller[1]['class'].'::'.$caller[1]['function'] : ''));
	}
	
	public static function output() {
		$sql_num = count(self::$log['sql']);
		echo '<tr><td colspan="3">'.$sql_num.' sql queries executed:</td></tr>';
		for($i = 0; $i < $sql_num; $i++) {
			echo '<tr><td>'.($i+1).'.</td><td>'.self::$log['sql'][$i].'</td><td>'.self::$log['sql_func'][$i][0].' <i>('.self::$log['sql_func'][$i][1].')</i>'.(self::$log['sql_func'][$i][2] ? '<b>'.self::$log['sql_func'][$i][2].'</b>' : '').'</td></tr>';
		}
	}
}
?>

test.php

<?php

// From : http://forum.opencart.com/viewtopic.php?t=27918

/*
1. Extract attached archive
 debug.zip
(493 Bytes) Downloaded 335 times
in /system/helper/ directory.
2. Open index.php file(located in root directory).
3. Add the following block at the beginning of the file:
*/

// debug hack start
define('DEBUG', true); // comment to disable debug mode
define('SQL_DEBUG', true); // comment to disable sql-debugging

if(defined('DEBUG')) {
   $start_time =  microtime();
   $start_mem = memory_get_usage();
}
// debug hack endd

// 4. Right after require_once('config.php'); line add:

// debug hack start
if(defined('SQL_DEBUG'))
   require_once(DIR_SYSTEM . 'helper/debug.php');
// debug hack end


// 5. And at the end of the file (after $response->output(); line) add:

// debug hack start
if(defined('DEBUG')) {
   $end_time = (microtime() - $start_time);
   $end_mem = (memory_get_usage() - $start_mem);
   echo '<style>#debug tr:hover {background:#FFC}</style><table id="debug" border="1" width="100%">';
   echo '<tr><td colspan="3">Memory Usage: '.round((($end_mem / 1024) / 1024), 6).' MB</td></tr>';
   echo '<tr><td colspan="3">Execution Time: '.abs(round($end_time, 6)).' seconds</td></tr>';
   if(defined('SQL_DEBUG'))
      Debug::output();
   echo '</table>';
}
// debug hack end


// 6. Open /system/library/db.php file and after line:

public function query($sql) {

// add the following line:

if(defined('SQL_DEBUG')) { $caller = debug_backtrace(); Debug::trigger('sql', $sql, $caller); }

//// ....

?>

something wrong with microtime(). i changed microtime() to time()