Output Buffering With PHP - The Real World
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So that's the theory - now how about tossing it into the real world and seeing how it works? Take a look at the following PHP script, which demonstrates how output buffering, together with a series of judicious calls to PHP's error-handling functions, can be used to provide a simple and efficient wrapper for catching and handling errors in a script.
<?php// use an output buffer to store page contentsob_start();?><html><head><basefont face="Arial"></head><body><h2>News</h2><?php// custom error handlerfunction e($type, $msg, $file, $line){ // read some environment variables // these can be used to provide some additional debuginformation 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, $errorLog); // discard current buffer contents // and turn off output buffering ob_end_clean(); // display error page echo "<html><head><basefont face=Arial></head><body>"; echo "<h1>Error!</h1>"; echo "We're sorry, but this page could not be displayed becauseof an internal error. The error has been recorded and will be rectifiedas soon as possible. Our apologies for the inconvenience. <p> <ahref=/>Click here to go back to the main menu.</a>"; echo "</body></html>"; // exit exit();}// report warnings and fatal errorserror_reporting(E_ERROR | E_WARNING);// define a custom handlerset_error_handler("e");// attempt a MySQL connection$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, $connection);// if resultset existsif (mysql_num_rows($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($row->content, 0, 150); ?>... <ahref=story.php?id=<?=$row->id?>>Read more</a></font> <p><?php }?></ul><?php}else{ echo "No stories available at this time";}// no errors occured// print buffer contentsob_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, the 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.
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