Recipe 20.11. Logging Debugging Information

20.11.1. Problem

You want to make debugging easier by adding statements to print out variables. But you want to be able to switch back and forth easily between production and debug modes.

20.11.2. Solution

Put a function that conditionally prints out messages based on a defined constant in a page included using the auto_prepend_file configuration setting. Save the following code to debug.php:

// turn debugging on define('DEBUG',true); // generic debugging function function pc_debug($message) {     if (defined('DEBUG') && DEBUG) {         error_log($message);     } }

Set the auto_prepend_file directive in php.ini or your site .htaccess file:


Now call pc_debug( ) from your code to print out debugging information:

$sql = 'SELECT color, shape, smell FROM vegetables'; pc_debug("[sql: $sql]"); // only printed if DEBUG is true $r = mysql_query($sql);

20.11.3. Discussion

Debugging code is a necessary side effect of writing code. There are a variety of techniques to help you quickly locate and squash your bugs. Many of these involve including scaffolding that helps ensure the correctness of your code. The more complicated the program, the more scaffolding needed. Fred Brooks, in The Mythical Man-Month (Addison-Wesley), guesses that there's "half as much code in scaffolding as there is in product." Proper planning ahead of time allows you to integrate the scaffolding into your programming logic in a clean and efficient fashion. This requires you to think out beforehand what you want to measure and record and how you plan on sorting through the data gathered by your scaffolding.

One technique for sifting through the information is to assign different priority levels to different types of debugging comments. Then the debug function prints information only if it's higher than the current priority level:

define('DEBUG',2); function pc_debug($message, $level = 0) {     if (defined('DEBUG') && ($level > DEBUG) {         error_log($message);     } } $sql = 'SELECT color, shape, smell FROM vegetables'; pc_debug("[sql: $sql]", 1); // not printed, since 1 < 2 pc_debug("[sql: $sql]", 3); // printed, since 3 > 2

Another technique is to write wrapper functions to include additional information to help with performance tuning, such as the time it takes to execute a database query:

function db_query($sql) {     if (defined('DEBUG') && DEBUG) {          // start timing the query if DEBUG is on          $DEBUG_STRING = "[sql: $sql]<br>\n";          $starttime = microtime(true);     }     $r = mysql_query($sql);     if (! $r) {         $error = mysql_error();         error_log('[DB: query @'.$_SERVER['REQUEST_URI']."][$sql]: $error");     } elseif (defined(DEBUG) && DEBUG) {         // the query didn't fail and DEBUG is turned on, so finish timing it         $endtime = microtime(true);         $elapsedtime = $endtime - $starttime;         $DEBUG_STRING .= "[time: $elapsedtime]<br>\n";         error_log($DEBUG_STRING);     }     return $r; }

Here, instead of just printing out the SQL to the error log, you also record the number of seconds it takes MySQL to perform the request. This lets you see if certain queries are taking too long. See Recipe 21.1 for more discussion of timing code execution and for a PHP 4compatible alternative to microtime(true).

Finally, you may also want to integrate PEAR's Log package, which provides an efficient framework for an abstracted logging system. PEAR Log predefines eight log levels: PEAR_LOG_EMERG, PEAR_LOG_ALERT, PEAR_LOG_CRIT, PEAR_LOG_ERR, PEAR_LOG_WARNING, PEAR_LOG_NOTICE, PEAR_LOG_INFO, and PEAR_LOG_DEBUG. The Log package provides a robust assortment of options for customizing error logging, including logging errors to SQLite and/or to a pop-up browser window.

20.11.4. See Also

Documentation on define( ) at, defined( ) at, and error_log( ) at; The Mythical Man-Month by Frederick P. Brooks (Addison-Wesley); main page for PEAR Log at

PHP Cookbook, 2nd Edition
PHP Cookbook: Solutions and Examples for PHP Programmers
ISBN: 0596101015
EAN: 2147483647
Year: 2006
Pages: 445

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