PHP：[错误]如果只有“error_reporting”可以隐藏或显示错误，为什么“display_errors”存在？

EDIT: My Environment is the following (this is relevant to the answer):

  • Microsoft IIS 10.0 Express
  • Windows 7 Professional SP1
  • PHP Version 5.6.16 x86 FastCGI

I am trying to figure out what purpose the setting "display_errors" has. In the following code, I am attempting to hide the PHP Notice that is thrown at line 9. Naturally one would think that simply setting

 display_errors = 'Off'

would accomplish this, but it seems to have no effect.

I am of the understanding that "error_reporting" lets you choose which errors are caught, and "display_errors" determines whether or not the error messages are echoed to the user. Am I mistaken? In most of my Google searches, I see people setting both

error_reporting = 0

and

display_errors = 'Off'

But, why even bother with

   display_errors = 'Off'

when it does not have an effect anyway?

My test code:

<?php
//ini_set("error_reporting", 0);
ini_set("display_errors", "Off");
ini_set("display_startup_errors", "Off");
date_default_timezone_set("America/Chicago");

echo "Hey<br />";

echo $myarr["hey"];

When I run this code, I see the following error message (this is a link to the image, since I don't have enough reputation to post it here):

http://i.imgur.com/mupV8BO.png

PHP: The error message generated for line 9 should be hidden from the user. Why does "display_errors" exist if it does nothing?

After trial and error and using ShiraNai7's explanations I have found that this is a bug in PHP on Microsoft IIS and FastCGI. See link to bug:

Bug #44729 display_errors = Off not respected

https://bugs.php.net/bug.php?id=44729

Yes, PHP errors and their configuration may seem a bit really weird if one isn't familiar with the inner workings. There is a lot going on.

display_errors

  • controls whether PHP's default error handler prints errors to the output as they happen
    • error codes suppresed by error_reporting will not printed
  • controls whether PHP's default exception handler prints uncaught exceptions to the output

log_errors

  • controls whether PHP's default error handler logs errors to error_log
    • error codes suppresed by error_reporting will not be logged
  • controls whether PHP's default exception handler logs exceptions to error_log

error_reporting

  • defines a global mask for errors that should be reported
  • this option is used by the PHP's default error handler (see above)
  • this option should be used by the custom error handler too

set_error_handler()

  • defines a custom error handler
  • it is always called if an error occurs (even if its suppressed)
  • if it returns false, the PHP's default error handler WILL be called (and print / log errors according to the settings)
  • if it returns null or true or throws an exception, the PHP's default error handler WILL NOT be called (and thus it won't print / log anything).

set_exception_handler()

  • defines a custom exception handler
  • PHP's default exception handler will not be called if a custom handler is defined (and thus it won't print / log anything)

But, why even bother with "display_errors = 'Off'" when it does not have an effect anyway?

There are combinations of the above settings that make display_errors not useless.

Examples:

  • display_errors must be off in php.ini on production servers to prevent leaking of possibly sensitive data
    • an error / exception can happen before you have the chance to change any of the other settings
  • if you want to log errors but not print them to the output, you set display_errors to off and log_errors to on
  • if you are using a custom error handler that prints custom error messages but you also want to use error_log, you set display_errors to off