The first part of this article demonstrated basic errorhandling in PHP, explaining the various error types and illustrating theprocess of building a custom error handler. But that's just the tip ofthe iceberg - this concluding part goes a step further, showing you totrigger your own errors, and log error messages to a file, database oremail address.
You might remember, from the first part of this article, an example which demonstrated how a custom error handler could be used to write error messages to a file on a PHP-driven Web site. You might also remember that one of the drawbacks of that script was the fact that warnings would get printed while the page was being constructed.
It's possible to bypass this problem - and also simplify the error logging mechanism used in that example - via a series of judicious calls to PHP's output-buffering functions. Take a look at this revised script, which sets things up just right:
<?php
// use an output buffer to store page contents
ob_start();
?>
<html>
<head><basefont face="Arial"></head>
<body>
<h2>News</h2>
<?php
// custom error handler
function e($type, $msg, $file, $line)
{
// read some environment variables
// these can be used to provide some additional debug
information
global $HTTP_HOST, $HTTP_USER_AGENT, $REMOTE_ADDR, $REQUEST_URI;
// define the log file
$errorLog = "error.log";
// construct the error string
$errorString = "Date: " . date("d-m-Y H:i:s", mktime()) . "\n";
$errorString .= "Error type: $type\n";
$errorString .= "Error message: $msg\n";
$errorString .= "Script: $file($line)\n";
$errorString .= "Host: $HTTP_HOST\n";
$errorString .= "Client: $HTTP_USER_AGENT\n";
$errorString .= "Client IP: $REMOTE_ADDR\n";
$errorString .= "Request URI: $REQUEST_URI\n\n";
// log the error string to the specified log file
error_log($errorString, 3, $errorLog);
// discard current buffer contents
// and turn off output buffering
ob_end_clean();
// display error page
echo "<html><head><basefont face=Arial></head><body>";
echo "<h1>Error!</h1>";
echo "We're sorry, but this page could not be displayed because
of an internal error. The error has ben recorded and will be rectified
as soon as possible. Our apologies for the inconvenience. <p> <a
href=/>Click here to go back to the main menu.</a>";
echo "</body></html>";
// exit
exit();
}
// report warnings and fatal errors
error_reporting(E_ERROR | E_WARNING);
// define a custom handler
set_error_handler("e");
// attempt a MySQL connection
$connection = @mysql_connect("localhost", "john", "doe");
mysql_select_db("content");
// generate and execute query
$query = "SELECT * FROM news ORDER BY timestamp DESC";
$result = mysql_query($query, $connection);
// if resultset exists
if (mysql_num_rows($result) > 0)
{
?>
<ul>
<?php
// iterate through query results
// print data
while($row = mysql_fetch_object($result))
{
?>
<li><b><?=$row->slug?></b>
<br>
<font size=-1><i><?=$row->timestamp?></i></font>
<p>
<font size=-1><?php echo substr($row->content, 0, 150); ?>... <a
href=story.php?id=<?=$row->id?>>Read more</a></font>
<p>
<?php
}
?>
</ul>
<?php
}
else
{
echo "No stories available at this time";
}
// no errors occured
// print buffer contents
ob_end_flush();
?>
</body>
</html>
In this case, the first thing I've done is initialized the
output buffer via a call to ob_start() - this ensures that all script output is placed in a buffer, rather than being displayed to the user. This output may be dumped to the standard output device at any time via a call to ob_end_flush().
Now, whenever an error occurs, my custom error handler, cleverly named e(), will first flush the output buffer, then send a custom error template to the browser and terminate script execution. So, even if there was a Web page being constructed on the fly when the error occurred, it will never see the light of day, as it will be discarded in favour of the custom error template. If, on the other hand, the script executes without any errors, the final call to ob_end_flush will output the fully-generated HTML page to the browser.
Note that, as before, fatal errors cannot be handled by the custom handler. The only way to avoid the output of fatal error messages is by telling PHP not to display them (take a look at the "display_errors" configuration directive in the PHP configuration file).
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