运算符
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错误控制运算符

PHP 支持一个错误控制运算符：@。当将其放置在一个 PHP 表达式之前，该表达式可能产生的任何错误信息都被忽略掉。

如果激活了 track_errors 特性，表达式所产生的任何错误信息都被存放在变量 $php_errormsg 中。此变量在每次出错时都会被覆盖，所以如果想用它的话就要尽早检查。

<?php
/* Intentional file error */
$my_file = @file ('non_existent_file') or
    die ("Failed opening file: error was '$php_errormsg'");

// this works for any expression, not just functions:
$value = @$cache[$key];
// will not issue a notice if the index $key doesn't exist.

?>

Note: @ 运算符只对表达式有效。对新手来说一个简单的规则就是：如果能从某处得到值，就能在它前面加上 @ 运算符。例如，可以把它放在变量，函数和 include() 调用，常量，等等之前。不能把它放在函数或类的定义之前，也不能用于条件结构例如 if 和 foreach 等。

参见 error_reporting() 及手册中错误处理及日志函数的有关章节。

Warning

目前的“@”错误控制运算符前缀甚至使导致脚本终止的严重错误的错误报告也失效。这意味着如果在某个不存在或类型错误的函数调用前用了“@”来抑制错误信息，那脚本会没有任何迹象显示原因而死在那里。

运算符
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PHP手册 - N: 错误控制运算符

用户评论:

bohwaz (22-Jun-2011 02:27)

If you use the ErrorException exception to have a unified error management, I'll advise you to test against error_reporting in the error handler, not in the exception handler as you might encounter some headaches like blank pages as error_reporting might not be transmitted to exception handler.

So instead of :

<?php

function exception_error_handler($errno, $errstr, $errfile, $errline )
{
    throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
}

set_error_handler("exception_error_handler");

function catchException($e)
{
    if (error_reporting() === 0)
    {
        return;
    }

    // Do some stuff
}

set_exception_handler('catchException');

?>

It would be better to do :

<?php

function exception_error_handler($errno, $errstr, $errfile, $errline )
{
    if (error_reporting() === 0)
    {
        return;
    }

    throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
}

set_error_handler("exception_error_handler");

function catchException($e)
{
    // Do some stuff
}

set_exception_handler('catchException');

?>

anthon at piwik dot org (20-Feb-2011 08:39)

If you're wondering what the performance impact of using the @ operator is, consider this example.  Here, the second script (using the @ operator) takes 1.75x as long to execute...almost double the time of the first script.

So while yes, there is some overhead, per iteration, we see that the @ operator added only .005 ms per call.  Not reason enough, imho, to avoid using the @ operator.

<?php
function x() { }
for ($i = 0; $i < 1000000; $i++) { x(); }
?>

real    0m7.617s
user    0m6.788s
sys    0m0.792s

vs

<?php
function x() { }
for ($i = 0; $i < 1000000; $i++) { @x(); }
?>

real    0m13.333s
user    0m12.437s
sys    0m0.836s

auser at anexample dot com (08-Sep-2010 12:02)

Be aware that using @ is dog-slow, as PHP incurs overhead to suppressing errors in this way. It's a trade-off between speed and convenience.

darren at powerssa dot com (14-Jul-2010 05:33)

After some time investigating as to why I was still getting errors that were supposed to be suppressed with @ I found the following.

1. If you have set your own default error handler then the error still gets sent to the error handler regardless of the @ sign.

2. As mentioned below the @ suppression only changes the error level for that call. This is not to say that in your error handler you can check the given $errno for a value of 0 as the $errno will still refer to the TYPE(not the error level) of error e.g. E_WARNING or E_ERROR etc

3. The @ only changes the rumtime error reporting level just for that one call to 0. This means inside your custom error handler you can check the current runtime error_reporting level using error_reporting() (note that one must NOT pass any parameter to this function if you want to get the current value) and if its zero then you know that it has been suppressed.
<?php
// Custom error handler
function myErrorHandler($errno, $errstr, $errfile, $errline)
{
    if ( 0 == error_reporting () ) {
        // Error reporting is currently turned off or suppressed with @
        return;
    }
    // Do your normal custom error reporting here
}
?>

For more info on setting a custom error handler see: http://php.net/manual/en/function.set-error-handler.php
For more info on error_reporting see: http://www.php.net/manual/en/function.error-reporting.php

M. T. (11-Oct-2009 05:20)

Be aware of using error control operator in statements before include() like this:

<?PHP

(@include("file.php"))
 OR die("Could not find file.php!");

?>

This cause, that error reporting level is set to zero also for the included file. So if there are some errors in the included file, they will be not displayed.

dsbeam at gmail dot com (02-Sep-2009 03:22)

Though error suppression can be dangerous at times, it can be useful as well.  I've found the following statements roughly equivalent:

     if( isset( $var ) && $var === $something )
     if( @$var === $something )

EXCEPT when you're comparing against a boolean value (when $something is false).  In that case, if it's not set the conditional will still be triggered.

I've found this useful when I want to check a value that might not exist:

     if( @$_SERVER[ 'HTTP_REFERER' ] !== '/www/some/path/file' )

or when we want to see if a checkbox / radio button have been submitted with a post action

     if( @$_POST[ 'checkbox' ] === 'yes' )

Just letting you guys know my findings, :)

gerrywastaken (19-May-2009 09:46)

Error suppression should be avoided if possible as it doesn't just suppress the error that you are trying to stop, but will also suppress errors that you didn't predict would ever occur. This will make debugging a nightmare.

It is far better to test for the condition that you know will cause an error before preceding to run the code. This way only the error that you know about will be suppressed and not all future errors associated with that piece of code.

There may be a good reason for using outright error suppression in favor of the method I have suggested, however in the many years I've spent programming web apps I've yet to come across a situation where it was a good solution. The examples given on this manual page are certainly not situations where the error control operator should be used.

taras dot dot dot di at gmail dot com (12-Aug-2008 04:29)

I was confused as to what the @ symbol actually does, and after a few experiments have concluded the following:

* the error handler that is set gets called regardless of what level the error reporting is set on, or whether the statement is preceeded with @

* it is up to the error handler to impart some meaning on the different error levels. You could make your custom error handler echo all errors, even if error reporting is set to NONE.

* so what does the @ operator do? It temporarily sets the error reporting level to 0 for that line. If that line triggers an error, the error handler will still be called, but it will be called with an error level of 0

Hope this helps someone

beatngu (27-May-2008 10:29)

NB The @ operator doesn't work when throwing errors as exceptions using the ErrorException class

nospam at blog dot fileville dot net (03-Jan-2007 07:58)

If you want to log all the error messages for a php script from a session you can use something like this:
<?php
 session_start();
  function error($error, $return=FALSE) {
      global $php_errormsg;
      if(isset($_SESSION['php_errors'])) {
        $_SESSION['php_errors'] = array();    
    }
  $_SESSION['php_errors'][] = $error; // Maybe use $php_errormsg
  if($return == TRUE) {
    $message = "";
       foreach($_SESSION['php_errors'] as $php_error) {
          $messages .= $php_error."\n";
     } 
    return $messages; // Or you can use use $_SESSION['php_errors']
  }
}
?>
Hope this helps someone...

(13-Dec-2006 01:52)

error_reporting()==0 for detecting the @ error suppression assumes that you did not set the error level to 0 in the first place.

However, typically if you want to set your own error handler, you would set the error_reporting to 0. Therefore, an alternative to detect the @ error suppression is required.

programming at kennebel dot com (13-Oct-2006 02:38)

To suppress errors for a new class/object:

<?php
// Tested: PHP 5.1.2 ~ 2006-10-13

// Typical Example
$var = @some_function();

// Class/Object Example
$var = @new some_class();

// Does NOT Work!
//$var = new @some_class(); // syntax error
?>

I found this most useful when connecting to a
database, where i wanted to control the errors
and warnings displayed to the client, while still
using the class style of access.

me at hesterc dot fsnet dot co dot uk (03-Mar-2005 04:25)

If you wish to display some text when an error occurs, echo doesn't work. Use print instead. This is explained on the following link 'What is the difference between echo and print?':

http://www.faqts.com/knowledge_base/view.phtml/aid/1/fid/40

It says "print can be used as part of a more complex expression where echo cannot".

Also, you can add multiple code to the result when an error occurs by separating each line with "and". Here is an example:

<?php
$my_file = @file ('non_existent_file') or print 'File not found.' and $string = ' Honest!' and print $string and $fp = fopen ('error_log.txt', 'wb+') and fwrite($fp, $string) and fclose($fp);
?>

A shame you can't use curly brackets above to enclose multiple lines of code, like you can with an if statement or a loop. It could make for a single long line of code. You could always call a function instead.

frogger at netsurf dot de (26-Dec-2004 04:19)

Better use the function trigger_error() (http://de.php.net/manual/en/function.trigger-error.php)
to display defined notices, warnings and errors than check the error level your self. this lets you write messages to logfiles if defined in the php.ini, output
messages in dependency to the error_reporting() level and suppress output using the @-sign.