Treating Warnings as Errors

See example #1 at http://www.php.net/manual/en/class.errorexception.php

<?php
function exception_error_handler($errno, $errstr, $errfile, $errline ) {
    throw new ErrorException($errstr, $errno, 0, $errfile, $errline);
}
set_error_handler("exception_error_handler");

/* Trigger exception */
strpos();
?>

Have a look at set_error_handler() and include this at the beginning of your scripts or in your bootstrap to just print the stacktrace when E_WARNINGs occur.

function stacktrace_error_handler($errno,$message,$file,$line,$context)
{
    if($errno === E_WARNING) {
        debug_print_backtrace();
    }
    return false; // to execute the regular error handler
}
set_error_handler("stacktrace_error_handler");

For more control over the various types, have a look at the more explicit version posted elsewhere in the answers.

You can define your own error handler using set_error_handler()

In the handler function you can treat each class of error however you want. Here's a basic template that I use, in my case I only want to handle fatal errors, so I ignore notices and warnings.

In your case you could do a backtrace on warnings, or log them however you want

function error_handler($errno,$message,$file,$line,$context) {

switch($errno) {
    // ignore warnings and notices
    case E_WARNING:
    case E_NOTICE:
    case E_USER_NOTICE:
    case E_USER_WARNING:
        break;
    // log PHP and user errors
    case E_ERROR:
    case E_USER_ERROR:
              // Do some processing on fatal errors
    }
}