为什么使用 set_error_handler(） 可以平息我的错误

我正在使用以下脚本处理错误：

<?php # config.inc.php
// This script establishes email default settings.
// This script determines how errors are handled.
// Email Settings
$site['from_name'] = 'x'; // from email name
$site['from_email'] = 'x@x.com'; // from email address
// Just in case we need to relay to a different server, 
// provide an option to use external mail server.
$site['smtp_mode'] = 'enabled'; // enabled or disabled
$site['smtp_host'] = 'mail.x.com';
$site['smtp_port'] = null;
$site['smtp_username'] = 'admin@x.com'; 
$site['smtp_password'] = 'x';
// Error handling:
// Flag variable for site status:
$live = TRUE;
ini_set('display_errors','On');
error_reporting(E_ALL);
// Error log email address:
$admin_email = 'x@x.com';
date_default_timezone_set('America/New_York');
// Create the error handler.
function my_error_handler ($e_number, $e_message, $e_file, $e_line, $e_vars) {
    global $live, $admin_email;
    // Build the error message.
    $message = "An error occurred in script '$e_file' on line $e_line: 'n<br />$e_message'n<br />";
    // Add the date and time.
    $message .= "Date/Time: " . date('n-j-Y H:i:s') . "'n<br />";
    // Append $e_vars to the $message.
    $message .= "<pre>" . print_r ($e_vars, 1) . "</pre>'n<br />";
    if ($live) { // Don't show the specific error.
        error_log ($message, 1, $admin_email); // Send email.
        // Only print an error message if the error isn't a notice.
        if ($e_number != E_NOTICE) {
            echo '<div id="Error">A system error occurred. An administrator has been notified. We apologize for the inconvenience.</div><br />';
        }
    } else { // Development (print the error).
        echo '<div id="Error">' . $message . '</div><br />';
    }
} // End of my_error_handler() definition.
// Use my error handler.
set_error_handler ('my_error_handler');
?>

包含在有错误的脚本中，则不会显示该错误。如果我注释掉对 set_error_handler(( 的调用，则会显示错误。我做错了什么可能导致这种行为？