_error_handler 函数

``` php
if ( ! function_exists('_error_handler'))
{
  /**
  * Error Handler
  *
  * This is the custom error handler that is declared at the (relative)
  * top of CodeIgniter.php. The main reason we use this is to permit
  * PHP errors to be logged in our own log files since the user may
  * not have access to server logs. Since this function effectively
  * intercepts PHP errors, however, we also need to display errors
  * based on the current error_reporting level.
  * We do that with the use of a PHP error template.
  *
  * @param  int $severity
  * @param  string  $message
  * @param  string  $filepath
  * @param  int $line
  * @return void
  */
  function _error_handler($severity, $message, $filepath, $line)
  {
    // @todo 已解决 - 如何进行的错误控制
    // $severity 是此 5 个错误等级中的一个时, 那么 is_error 为真
    // E_ERROR: 1(1), E_PARSE: 4(100), E_COMPILE_ERROR: 64(1000000), E_CORE_ERROR: 16(10000), E_USER_ERROR: 256(100000000)
    $is_error = (((E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR) & $severity) === $severity);

    // When an error occurred, set the status header to '500 Internal Server Error'
    // to indicate to the client something went wrong.
    // This can't be done within the $_error->show_php_error method because
    // it is only called when the display_errors flag is set (which isn't usually
    // the case in a production environment) or when errors are ignored because
    // they are above the error_reporting threshold.
    if ($is_error)
    {
      set_status_header(500);
    }

    // Should we ignore the error? We'll get the current error_reporting
    // level and add its bits with the severity bits to find out.
    if (($severity & error_reporting()) !== $severity)
    {
      return;
    }

    $_error =& load_class('Exceptions', 'core');
    $_error->log_exception($severity, $message, $filepath, $line);

    // Should we display the error?
    if (str_ireplace(array('off', 'none', 'no', 'false', 'null'), '', ini_get('display_errors')))
    {
      $_error->show_php_error($severity, $message, $filepath, $line);
    }

    // If the error is fatal, the execution of the script should be stopped because
    // errors can't be recovered from. Halting the script conforms with PHP's
    // default error handling. See http://www.php.net/manual/en/errorfunc.constants.php
    if ($is_error)
    {
      exit(1); // EXIT_ERROR
    }
  }
}
```

```
// $severity 是此 5 个错误等级中的一个时, 那么 is_error 为真
// E_ERROR: 1(1), E_PARSE: 4(100), E_COMPILE_ERROR: 64(1000000), E_CORE_ERROR: 16(10000), E_USER_ERROR: 256(100000000)
$is_error = (((E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR) & $severity) === $severity);
```

其中 E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR 得到值为 341(101010101)

101010101 & $severity 对传进来的 $severity 进行位与运算.

如果结果和自身相等(即 $severity 等于 E_ERROR | E_PARSE | E_COMPILE_ERROR | E_CORE_ERROR | E_USER_ERROR 中的某一个) 那么 $is_error 为真

接下来跟系统的错误等级比较, 判断是否要显示错误

如果需要显示, 那么就会记录日志, 显示错误, 停止继续执行.