Mage_Core_Model_App.php

<?php
class Mage_Core_Model_App
{
// ...
    /**
     * Redeclare custom error handler
     *
     * @param   string $handler
     * @return  Mage_Core_Model_App
     */
    public function setErrorHandler($handler)
    {
        if (!Mage::getIsDeveloperMode()) {
            set_error_handler($handler);
        } else {
            require_once __DIR__ . '/../../../../../../lib/PHP-Error/php_error.php';
            \php_error\reportErrors();
        }
        return $this;
    }
// ...