Lets say you have a relatively complex php web application, like wordpress. You have it running under apache (which is common.) You have good control of your site via .htaccess (which is common — permalinks and all.) And something happens to your blog (e.g. someone is exploiting some unknown vulnerability to compromise your content), which you want to track down. So you want to log, for instance, HTTP POST data. Your first instinct might be to add some logging code to index.php (mine was) But there are a lot of possible places which might be directly accessed, especially in the admin. So The trick I use (and this is probably the only time I’ve ever condoned this) is to use PHPs auto_prepend_file functionality.

Make a /home/user/postlog/ directory, then a /home/user/postlog/logs/ directory (and chmod a+rw that one.) Next make a simple /home/user/postlog/postlog.php file with the following contents:

<?php 
if ( isset($_POST) && is_array($_POST) && count($_POST) > 0 ) { 
  $log_dir = dirname( __FILE__ ) . '/logs/'; 
  $log_name = "posts-" . $_SERVER['REMOTE_ADDR'] . "-" . date("Y-m-d-H") . ".log"; 
  $log_entry = gmdate('r') . "\t" . $_SERVER['REQUEST_URI'] . "
" . serialize($_POST) . "

"; 
  $fp=fopen( $log_dir . $log_name, 'a' ); 
  fputs($fp, $log_entry); 
  fclose($fp); } 
?>

Finally add this line to the top of your .htaccess file:

php_value auto_prepend_file /home/user/postlog/postlog.php

If all went well this should start logging any request to any php file with any post data into the /home/user/postlog/logs/ direcory (with a unique log per ip per day)