PHP: 错误控制运算符

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诊断都被抑制。

如果用 set_error_handler() 设定了自定义的错误处理函数，即使诊断信息被抑制，也仍然会被调用。

警告

error_get_last() 返回数组中的 "message" 元素储存了表达式产生的任意错误信息。此函数的返回结果会随着每次错误的发生而相应变化，所以需要尽早检查。

<?php
/* 故意文件错误 */
$my_file = @file ('non_existent_file') or
    die ("Failed opening file: error was '" . error_get_last()['message'] . "'");

// 这适用于所有表达式，而不仅仅是函数：
$value = @$cache[$key];
// 如果索引 $key 不存在，则不会发出通知。

?>

注意: @ 运算符只对表达式有效。对新手来说一个简单的规则就是：如果能从某处获得值，就能在它前面加上 @ 运算符。例如，可以把它放在变量，函数调用，某些语言构造调用（例如 include ）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。

警告

PHP 8.0.0 之前，@ 运算符会禁用导致脚本停止运行的严重错误。例如在调用一个不存在的函数前添加 @，由于函数不可用或者输入错误，将会导致脚本终止，而不会说明原因。

参见

＋add a note

User Contributed Notes 24 notes

down

206

taras dot dot dot di at gmail dot com ¶

15 years ago

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

down

127

M. T. ¶

14 years ago

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.

down

Anonymous ¶

10 years ago

This operator is affectionately known by veteran phpers as the stfu operator.

down

anthon at piwik dot org ¶

12 years ago

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

down

gerrywastaken ¶

14 years ago

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.

down

jcmargentina at gmail dot com ¶

4 years ago

Please be aware that the behaviour of this operator changed from php5 to php7.

The following code will raise a Fatal error no matter what, and you wont be able to suppress it

<?php

function query()
{
    $myrs = null;
    $tmp = @$myrs->free_result();

    return $tmp;
}

var_dump(query());

echo "THIS IS NOT PRINT";
?>

more info at: https://bugs.php.net/bug.php?id=78532&thanks=3

down

dkellner ¶

7 years ago

There is no reason to NOT use something just because "it can be misused".  You could as well say "unlink is evil, you can delete files with it so don't ever use unlink".

It's a valid point that the @ operator hides all errors - so my rule of thumb is: use it only if you're aware of all possible errors your expression can throw AND you consider all of them irrelevant.

A simple example is
<?php

    $x = @$a["name"];

?>
There are only 2 possible problems here: a missing variable or a missing index.  If you're sure you're fine with both cases, you're good to go.  And again: suppressing errors is not a crime.  Not knowing when it's safe to suppress them is definitely worse.

down

man13or at hotmail dot fr ¶

4 years ago

Quick debugging methods :

@print($a);
is equivalent to
if isset($a) echo $a ;

@a++;
is equivalent to
if isset($a) $a++ ;
else $a = 1;

down

Ryan C ¶

2 years ago

It's still possible to detect when the @ operator is being used in the error handler in PHP8. Calling error_reporting() will no longer return 0 as documented, but using the @ operator does still change the return value when you call error_reporting().

My PHP error settings are set to use E_ALL, and when I call error_reporting() from the error handler of a non-suppressed error, it returns E_ALL as expected.

But when an error occurs on an expression where I tried to suppress the error with the @ operator, it returns: E_ERROR | E_PARSE | E_CORE_ERROR | E_COMPILE_ERROR | E_USER_ERROR | E_RECOVERABLE_ERROR (or the number 4437). 

I didn't want to use 4437 in my code in case it changes with different settings or future versions of PHP, so I now use:

<?php
   function my_error_handler($err_no, $err_msg, $filename, $linenum) {
      if (error_reporting() != E_ALL) {
         return false; // Silenced
      }

     // ...
   }
?>

If the code needs to work with all versions of PHP, you could check that error_reporting() doesn't equal E_ALL or 0.

And, of course, if your error_reporting settings in PHP is something other than E_ALL, you'll have to change that to whatever setting you do use.

down

darren at powerssa dot com ¶

13 years ago

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

down

auser at anexample dot com ¶

13 years ago

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.

down

bohwaz ¶

12 years ago

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');

?>

down

programming at kennebel dot com ¶

17 years ago

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.

down

frogger at netsurf dot de ¶

18 years ago

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.

down

karst dot REMOVETHIS at onlinq dot nl ¶

8 years ago

While you should definitely not be too liberal with the @ operator, I also disagree with people who claim it's the ultimate sin.

For example, a very reasonable use is to suppress the notice-level error generated by parse_ini_file() if you know the .ini file may be missing.
In my case getting the FALSE return value was enough to handle that situation, but I didn't want notice errors being output by my API.

TL;DR: Use it, but only if you know what you're suppressing and why.

down

ricovox ¶

6 years ago

What is PHP's behavior for a variable that is assigned the return value of an expression protected by the Error Control Operator when the expression encounteres an error? 

Based on the following code, the result is NULL (but it would be nice if this were confirmed to be true in all cases).

<?php 

    $var = 3; 
    $arr = array(); 

    $var = @$arr['x'];    // what is the value of $var after this assignment?

    // is it its previous value (3) as if the assignment never took place?
    // is it FALSE or NULL?
    // is it some kind of exception or error message or error number?

   var_dump($var);  // prints "NULL"

?>

down

gwyneth dot llewelyn at gwynethllewelyn dot net ¶

2 years ago

Allegedly, the @ error control operator has been deprecated as of PHP 8.0.0, or, more precisely, it will no longer suppress fatal errors. See https://bugs.php.net/bug.php?id=78532&thanks=3 (note the comment by nikic@php.net)

down

fy dot kenny at gmail dot com ¶

3 years ago

* How to make deprecated super global variable `$php_errormsg` work

>1. modify php.ini 
>track_errors = On
>error_reporting = E_ALL & ~E_NOTICE
>2. Please note，if you already using customized error handler，it will prompt `undefined variable`
>please insert code`set_error_handler(null);` before executing code, e.g: 
>```php
>set_error_handler(null);
>$my_file = @file ('phpinfo.phpx') or  
>die ("<br>Failed opening file: <br>\t$php_errormsg");
>```

>(c)Kenny Fang

down

manisha at mindfiresolutions dot com ¶

9 years ago

Prepending @ before statement like you are doing a crime with yourself.

down

-1

Joey ¶

5 years ago

In PHP it is extremely beneficial to turn all notices into exceptions. This helps in creating bug free code through finding errors sooner rather than later. It also helps reduce the impact of bugs as code when entering an erroneous state ends sooner rather than later and at all. In the worst case without that you can have much more scenarios where code fails yet appears as though it succeeded.

However there are rare cases in which notices and warnings are produced where the above behavour might be unproductive. Worse yet your error handling will kick out that exception before the function gets to return.

They are rare cases such as socket handling where certain states are expressed through errors causing ambiguity.

down

-5

Anonymous ¶

9 years ago

I was wondering if anyone (else) might find a directive to disable/enable to error operator would be a useful addition. That is, instead of something like (which I have seen for a few places in some code):

<?php

if (defined(PRODUCTION)) {
    @function();
}
else {
    function();
}

?>

There could be something like this:

<?php

if (defined(PRODUCTION)) {
    ini_set('error.silent',TRUE);
}
else {
    ini_set('error.silent',FALSE);
}

?>

down

-7

nospam at blog dot fileville dot net ¶

16 years ago

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...

down

-10

Anonymous ¶

16 years ago

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.

down

-15

me at hesterc dot fsnet dot co dot uk ¶

18 years ago

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.

＋add a note