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Reference Guide: What does this symbol mean in PHP? (PHP Syntax)
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Closed 4 years ago.
What does the '#' symbol do in the following code?
#mkdir(ROOT. "cache/");
It suppresses errors from displaying:
PHP supports one error control operator: the at sign (#). When prepended to an expression in PHP, any error messages that might be generated by that expression will be ignored.
If the track_errors feature is enabled, any error message generated by the expression will be saved in the variable $php_errormsg. This variable will be overwritten on each error, so check early if you want to use it.
As noted in the comments, I too cannot imagine a reason to actually use this functionality -- write code that deals appropriately with error states/conditions.
As pointed out, it is the error suppression operator.
But what has not been pointed out, is that it is very bad practice to use - errors should not fail silently.
Check for error returns, and use try/catch blocks where exceptions are being used.
In the specific example...
#mkdir(ROOT. "cache/");
...it ignores any errors from mkdir(). The docs says it returns FALSE on failure, so you should be doing...
if ( ! mkdir(ROOT. "cache/")) {
// Handle error.
}
People seem to forget that PHP was a quick dirty language for getting things done, only recently has it tried to be mature and sophisticated.
Error suppression is a quick and dirty way of making functions behave the way you need them to, because in web-development you cannot predict what will be thrown at you, and sometimes it is not worth caring!
A classic example is the useful function getimagesize, that allows you to get some information about an image that someone has uploaded.
This function chucks a wobbly if the image file is not a standard image file. It is not really the developers role to inspect a file, determine if it can be loaded into getimagesize. There might be elegant ways of doing this, but seriously I don't care!
just do this:
if( !($a = #getimagesize( $_FILE['file']['tmp_name'] )))
{
unlink( $_FILE['file']['tmp_name'] );
//politely tell user that you rejected their image!
}
yes, you could use try and catch statements which are more elegant, but in the end, you have caught the error and suppressed the error message, which is what you wanted without wearing out the tab-key!
Contrary to what above answers say, the # prefix used carefully does not result in a runaway train wreck. It just allows the developer to accommodate errors in the way they prefer.
Related
So I'm working on cleanup of a horrible codebase, and I'm slowly moving to full error reporting.
It's an arduous process, with hundreds of notices along the lines of:
Notice: Undefined index: incoming in /path/to/code/somescript.php on line 18
due to uses of variables assuming undefined variables will just process as false, like:
if($_SESSION['incoming']){
// do something
}
The goal is to be able to know when a incorrectly undefined variable introduced, the ability to use strict error/notice checking, as the first stage in a refactoring process that -will- eventually include rewriting of the spots of code that rely on standard input arrays in this way. There are two ways that I know of to replace a variable that may or may not be defined
in a way that suppresses notices if it isn't yet defined.
It is rather clean to just replace instances of a variable like $_REQUEST['incoming'] that are only looking for truthy values with
#$_REQUEST['incoming'].
It is quite dirty to replace instances of a variable like $_REQUEST['incoming'] with the "standard" test, which is
(isset($_REQUEST['incoming'])? $_REQUEST['incoming'] : null)
And you're adding a ternary/inline if, which is problematic because you can actually nest parens differently in complex code and totaly change the behavior.
So.... ...is there any unacceptable aspect to use of the # error suppression symbol compared to using (isset($something)? $something : null) ?
Edit: To be as clear as possible, I'm not comparing "rewriting the code to be good" to "#", that's a stage later in this process due to the added complexity of real refactoring. I'm only comparing the two ways (there may be others) that I know of to replace $undefined_variable with a non-notice-throwing version, for now.
Another option, which seems to work well with lame code that uses "superglobals" all over the place, is to wrap the globals in dedicated array objects, with more or less sensible [] behaviour:
class _myArray implements ArrayAccess, Countable, IteratorAggregate
{
function __construct($a) {
$this->a = $a;
}
// do your SPL homework here: offsetExists, offsetSet etc
function offsetGet($k) {
return isset($this->a[$k]) ? $this->a[$k] : null;
// and maybe log it or whatever
}
}
and then
$_REQUEST = new _myArray($_REQUEST);
This way you get back control over "$REQUEST" and friends, and can watch how the rest of code uses them.
You need to decide on your own if you rate the # usage acceptable or not. This is hard to rate from a third party, as one needs to know the code for that.
However, it already looks like that you don't want any error suppression to have the code more accessible for you as the programmer who needs to work with it.
You can create a specification of it in the re-factoring of the code-base you're referring to and then apply it to the code-base.
It's your decision, use the language as a tool.
You can disable the error suppression operator as well by using an own callback function for errors and warnings or by using the scream extension or via xdebug's xdebug.scream setting.
You answered you question yourself. It suppress error, does not debug it.
In my opinion you should be using the isset() method to check your variables properly.
Suppressing the error does not make it go away, it just stops it from being displayed because it essentially says "set error_reporting(0) for this line", and if I remember correctly it would be slower than checking isset() too.
And if you don't like the ternary operator then you should use the full if else statement.
It might make your code longer but it is more readable.
I would never suppress errors on a development server, but I would naturally suppress errors on a live server. If you're developing on a live server, well, you shouldn't. That means to me that the # symbol is always unacceptable. There is no reason to suppress an error in development. You should see all errors including notices.
# also slows things down a bit, but I'm not sure if isset() is faster or slower.
If it is a pain to you to write isset() so many times, I'd just write a function like
function request($arg, $default = null) {
return isset($_REQUEST[$arg]) ? trim($_REQUEST[$arg]) : $default;
}
And just use request('var') instead.
Most so-called "PHP programmers" do not understand the whole idea of assigning variables at all.
Just because of lack of any programming education or background.
Well, it isn't going a big deal with usual php script, coded with considerable efforts and consists of some HTML/Mysql spaghetti and very few variables.
Another matter is somewhat bigger code, when writing going to be relatively easy but debugging turns up a nightmare. And you are learn to value EVERY bloody error message as you come to understanding that error messages are your FRIENDS, not some irritating and disturbing things, which better to be gagged off.
So, upon this understanding you're learn to leave no intentional errors in your code.
And define all your variables as well.
And thus make error messages your friends, telling you that something gone wrong, lelping to hunt down some hard-spotting error which caused by uninitialized variable.
Another funny consequence of lack of education is that 9 out of 10 "PHP programmers" cannot distinguish error suppression from turning displaying errors off and use former in place of latter.
I've actually discovered another caveat of the # beyond the ones mentioned here that I'll have to consider, which is that when dealing with functions, or object method calls, the # could prevent an error even through the error kills the script, as per here:
http://us3.php.net/manual/en/language.operators.errorcontrol.php
Which is a pretty powerful argument of a thing to avoid in the rare situation where an attempt to suppress a variable notice suppressed a function undefined error instead (and perhaps that potential to spill over into more serious errors is another unvoiced reason that people dislike #?).
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Closed 11 years ago.
Possible Duplicate:
Reference - What does this symbol mean in PHP?
I'm making a web application that uses URL queries to access different parts of the application. I was looking for a solution to make an invalid query like index.php?page=dashboarrrd display an error 404 message instead of a PHP error.
After some searching, I found that I could use something like the following to do the job:
if(!#include($fileName)){
#include("pageData/404.php");
}
And that makes sense, but I don't know why that works. I mean, what the heck does the # before the include mean? I totally understand include $filename; but I need an explanation for #include ($fileName)
the code you really need is
$fileName = "pagedata/".basename($_GET['page']).".php";
if(is_readable($fileName)) {
include($fileName);
} else {
include("pagedata/404.php");
}
and # has absolutely nothing to do here
# is one of biggest delusions coming from lack of experience.
Ones who using it do expect only one kind of error, while in fact there can be many more. And to gag ALL possible messages to suppress only one of them is definitely like to throw out the child along with the bath.
There is a fundamental problem that makes such misunderstanding so widespread:
Most PHP users cannot distinguish three sides of error control:
error handling
error reporting
user notification.
Most of time in sake of [3] people mess with (1) and (2). While each of them require separate treatment:
your program should raise no intentional errors. No error should be part of program logic. All errors that ever raised should be only unexpected ones.
if you expect some error, you have to handle it. Not gag with #, but gracefully handle. is_readable() in my code exactly for that.
error reporting is for the programmer and should be always at max. So, error logging should be enabled on a live site and a programmer have to check all errors occurred. And of course he would be interested in such errors, thus # will do only harm here.
User-level error messages should be different from system ones. Your 404.php is a good example of such user-friendly behavior. As for the system error messages, a user shouldn't be able to see them at all. Just turn display_errors off and see - there is no use for the # again!
This is the # Error Control Operator (quoting) :
When prepended to an expression in
PHP, any error messages that might be
generated by that expression will be
ignored.
In normal conditions, if include cannot load the file you've passed as a parameter, it'll emit a warning.
Prepending the # operator to include will prevent that warning from being emited -- and, so, from being displayed / logged.
So, the following portion of code :
include 'does-not-exist.php';
Will get you the following warnings :
Warning: include(does-not-exist.php) [function.include]: failed to open stream: No such file or directory
Warning: include() [function.include]: Failed opening 'does-not-exist.php' for inclusion
While this line :
#include 'does-not-exist.php';
Will get you not warning.
And, as a sidenote, for information : Five reasons why the shut-op operator (#) should be avoided
The # suppresses errors. This is generally discouraged, as when developing you want to see errors.
Errors are easy to suppress when moving to a production environment with the display_errors setting to off. So yea, in most cases, there really is no need for the error to be suppressed.
EDIT
As an extra tidbit to "improve" that, what I used to do when dynamically including a file, is have an array which acts as a "white list" of valid requests. This does not "have" to be an array, just what I chose to do an example with.
$whiteList = array('filename1', 'index', 'home', 'about');
if (in_array($filename, $whiteList)) {
include($filename);
}else {
include('page/404.php');
}
This would do a few things, 1 make you not need the error suppressor. Two, it would make it a bit more securer, as without this, you would need to do a basename call to filter the text to prevent certain type of include injections etc. (Not knowing if you did this already, just extra information).
So yea, you may want analyze / look at other ways to achieve this and above is just one method :)
The use of "#" simply suppresses the error that would normally result from (in this instance) a missing file. Whilst generally its use is a very bad idea, there are some rare exceptions, such as the code snippet you provide above.
For more information, see the Error Control Operators section of the PHP manual.
Additionally, you might find the existing Reference - What does this symbol mean in PHP? question worthy of a quick scan.
The # in php suppresses all error output. For instance, if you had error reporting for warnings, an # in front of a function that generated a warning would not display the warning text.
include is an example of such a construct. If the included file is not found, it will display a warning saying so. The # is not necessary in the code at all, it is just there so that the user will not see warnings.
However, it is better to use apache (or php if you prefer) to change ini for displaying errors on the development site and not displaying them on the production site. That would make the # symbol useless.
A better question is why you need to do this 404 include. Why are you including a file for display? Why not have apache handle 404 redirects on its own? Why wouldn't the file exist in the first place?
# suppresses error messages. The parentheses are optional in include, but whoever wrote that snippet included them.
#include() is the opposite of require(). The first will silently ignore an (optional and missing) include script, while the second will throw an error and halt the script when the (critical) dependency is missing.
In this instance it is only senseful within the if(). The second should preferrably not have an error suppression, as it doesn't mask any seriously security-relevant error message.
There are many functions (well most really) in the PHP language that get all upset and throw warnings and notices when they don't like something about their input - rather than just returning FALSE (though they do that too).
One place this is really common is in the GD and string functions. They are very particular about their arguments and it's really easy for user input to fail meeting their standards.
For example, a user uploads a image that is corrupt (intentionally or unintentionally). Resulting in warnings from the GD library.
So far there are only three ways I have found to silence PHP on this issue:
Change your error reporting setting in the ini or at runtime (yuck).
Suppress errors with the slow # symbol.
Change error reporting right before/after the function:
like so:
$errorlevel=error_reporting();
error_reporting($errorlevel & ~E_NOTICE);
//...code that generates notices
error_reporting($errorlevel);
Naturally, the second two choices just make me sick. Which leaves me using 1) and toning down the PHP error settings. However, I want PHP to be in strict mode so that while I'm working I can catch logic bugs and bad form that might creep into my code. However, I don't want to have random errors thrown when PHP doesn't like something.
So is there any way to separate errors that are from malformed arguments (bad input) from errors that are from bad programming? For example:
If a user image is invalid just return FALSE and I'll deal with it. I don't need warnings.
If my passing of an image resource to print function is invalid throw warnings.
There's another alternative - use set_error_handler(), you can even call it just before the GD function call and return to the default with restore_error_handler().
There's a good comment in the question try and catch a warning that gives more detail on how this is accomplished.
This is an ill-design from the beginnings of PHP. A modern PHP library would throw an exception on error. And an exception may be caught. But back then GD was written PHP didn't yet support exceptions.
Thus I think that in this case it is legitimate to use the # operator.
Fix the code
Sanitize the input before you call the functions. This saves you errors.
There is no generic approach for all libraries besides editing the offending library and changing how it throws exceptions. Honeslty, a lot of PHP devs really didn't care about exceptions being thrown, but now more and more people are getting on the E_STRICT wagon. When the GD libs were generated the mentality probably was that throwing uncatchable errors wasn't that big a deal.
As far as the validating images with GD. The only thing you can really do is to use a different library or function to validate your images. You might try using magic byte functions to check if the images have proper headers(though this doesn't mean the rest of the file is structured correctly). At least using the magic byte functions will take care of obvious things like someone uploading a text file instead of a JPEG.
If there is something like that, you can use the #-operator
Its not a clean solution, but there are situations, where it's indispensable.
Did you check the image before using getimagesize() ?
However, I don't want to have random
errors thrown when PHP doesn't like
something.
Why not? PHP is trying to tell you something notable with warnings and notices. In the GD example you'll want to log when a user is uploading a corrupt file - especially if it's being used in an attack. Turn of the display of error messages and log everything.
I've seen code samples that use an # before fopen as in
$fh = #fopen($myFile, 'w');
What's the significance of this #?
It suppresses errors an expression may display.
You can read more about it here.
Example:
file_get_contents('file_does_not_exist'); //this shows an error
#file_get_contents('file_does_not_exist'); //this does not
Its PHP's error control char.
PHP supports one error control operator: the at sign (#). When prepended to an expression in PHP, any error messages that might be generated by that expression will be ignored.
More here.
Using # is always bad practice. It will suppress an error message if occurrs, while error messages are extremely useful for the programmer and suppressing it is suicide. PHP's fate is to be used mostly not by programmers but by casual users who have no clue. You've got this code from one of that latter kind. So, better to get rid of all #, so, you'll be able to see what happened and correct a mistake.
Note that every error message has it's particular meaning and explain what the problem is.
For example there can be filesystem permissions problem or PHP OPEN_BASEDIR setting to prevent file from open. So, an error message will tell you what to do. Error messages is good and # is evil.
I am currently refactoring some code for work and I have come across some function calls prefixed by the "#" symbol. As I understand it, this is intended to escape PHP error reporting if the call fails.
Is this type of thing good practice? I understand the rationale in a development environment but when the site is pushed to production shouldn't all errors be handled properly rather than just escaped?
The use of this symbol would therefore mean that the developer has to sort through the code at a later stage to remove all error reporting escapes.
I am unsure whether to remove these symbols and just find a better way to handle potential errors or not.
For clarity, the function this was used on was the native PHP fsockopen() function.
That's probably among the worst practices you can come across in php code. It basically tells the interpreter to suppress errors and just try to do whatever the code asks it to do regardless of the outcome.
A great way to drag yourself and fellow teammates into all-nighter phantom bug hunts once the app has grown substantially.
Try-catch with custom exception handling is the way to go.
I think it is sometimes understandable to use # for calling functions like fsockopen(), because when they fail they will raise a warning as well as returning false.
There may be cases where you expect these calls to fail regularly and therefore do not want a warning to be raised. Obviously you shouldn't be displaying warnings in production and should be logging them instead, but you might still want to use the # operator to stop your logs getting full. You could stop warnings getting reported at all by changing the error_reporting setting but that is not ideal.
That's called the error control operator, and is generally a very scary thing to consider using. A warning from the manual (the emboldening is mine):
Currently the "#" error-control
operator prefix will even disable
error reporting for critical errors
that will terminate script execution.
Among other things, this means that if
you use "#" to suppress errors from a
certain function and either it isn't
available or has been mistyped, the
script will die right there with no
indication as to why.
Using the "#" operator is very useful when you know that the function call can fail, like, for example, the fsockopen call. Best practice is to use this only when the function you are calling often fails and is a valid case in your application. Also, you should definitely check the return value of the function after calling it:
$fp = #fsockopen($hostname, $port);
if ($fp === false) {
// handle connection failure
}
else {
// handle connection success
}
You should avoid two things:
Not checking the return value;
Using the "#" operator where you don't expect an error -- for example when opening a local file or sending headers. When opening a local file fails, that is an error and it should be handled properly.
Note: you might also want to look at set_error_handler()
if you use your custom error-handlers, the # operator will not help you,
you will always get error-events from situations where your are handling the "Warning" in your code ... like at fsockopen etc.
so you can simple suppress effectively the warning this way:
function renameWithOutExpectedAndSelfHandledErrors( ... ) {
set_error_handler(function(){}); // deactivate all errors
$result = rename('not existing','blafussel');
restore_error_handler(); // restore old error-situation
return $result;
}