'At' symbol before variable name in PHP: @$_POST

I've seen function calls preceded with an at symbol to switch off warnings. Today I was skimming some code and found开发者_开发知识库 this:

$hn = @$_POST['hn'];

What good will it do here?

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The @ is the error suppression operator in PHP.

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.

See:

• Error Control Operators
• Bad uses of the @ operator

Update:

In your example, it is used before the variable name to avoid the E_NOTICE error there. If in the $_POST array, the hn key is not set; it will throw an E_NOTICE message, but @ is used there to avoid that E_NOTICE.

Note that you can also put this line on top of your script to avoid an E_NOTICE error:

error_reporting(E_ALL ^ E_NOTICE);

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It won't throw a warning if $_POST['hn'] is not set.

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All that means is that, if $_POST['hn'] is not defined, then instead of throwing an error or warning, PHP will just assign NULL to $hn.

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It suppresses warnings if $_POST['something'] is not defined.

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I'm answering 11 years later for completeness regarding modern php.

Since php 7.0, the null coalescing operator is a more straightforward alternative to silencing warnings in that case. The ?? operator was designed (among other things) for that purpose.

Without @, a warning is shown:

$ php -r 'var_dump($_POST["hn"]);'
PHP Warning:  Undefined array key "hn" in Command line code on line 1
NULL

The output with silencing warnings (@):

$ php -r 'var_dump(@$_POST["hn"]);'
NULL

Obtaining the same result with the modern null coalescing operator (??):

$ php -r 'var_dump($_POST["hn"] ?? null);'
NULL