In where shall I use isset() and !empty()

问题

I read somewhere that the isset() function treats an empty string as TRUE, therefore isset() is not an effective way to validate text inputs and text boxes from a HTML form.

So you can use empty() to check that a user typed something.

Is it true that the isset() function treats an empty string as TRUE?
Then in which situations should I use isset()? Should I always use !empty() to check if there is something?

For example instead of

if(isset($_GET['gender']))...

Using this

if(!empty($_GET['gender']))...

回答1:

isset vs. !empty

FTA:

"isset() checks if a variable has a value including (False, 0 or empty string), but not NULL. Returns TRUE if var exists; FALSE otherwise.

On the other hand the empty() function checks if the variable has an empty value empty string, 0, NULL or False. Returns FALSE if var has a non-empty and non-zero value."

回答2:

In the most general way :

isset tests if a variable (or an element of an array, or a property of an object) exists (and is not null)
empty tests if a variable (...) contains some non-empty data.

To answer question 1 :

$str = '';
var_dump(isset($str));

gives

boolean true

Because the variable $str exists.

And question 2 :

You should use isset to determine whether a variable exists ; for instance, if you are getting some data as an array, you might need to check if a key isset in that array.
Think about $_GET / $_POST, for instance.

Now, to work on its value, when you know there is such a value : that is the job of empty.

回答3:

Neither is a good way to check for valid input.

isset() is not sufficient because – as has been noted already – it considers an empty string to be a valid value.
! empty() is not sufficient either because it rejects '0', which could be a valid value.

Using isset() combined with an equality check against an empty string is the bare minimum that you need to verify that an incoming parameter has a value without creating false negatives:

if( isset($_GET['gender']) and ($_GET['gender'] != '') )
{
  ...
}

But by "bare minimum", I mean exactly that. All the above code does is determine whether there is some value for $_GET['gender']. It does not determine whether the value for $_GET['gender'] is valid (e.g., one of ("Male", "Female","FileNotFound")).

For that, see Josh Davis's answer.

回答4:

isset is intended to be used only for variables and not just values, so isset("foobar") will raise an error. As of PHP 5.5, empty supports both variables and expressions.

So your first question should rather be if isset returns true for a variable that holds an empty string. And the answer is:

$var = "";
var_dump(isset($var));

The type comparison tables in PHP’s manual is quite handy for such questions.

isset basically checks if a variable has any value other than null since non-existing variables have always the value null. empty is kind of the counter part to isset but does also treat the integer value 0 and the string value "0" as empty. (Again, take a look at the type comparison tables.)

回答5:

If you have a $_POST['param'] and assume it's string type then

isset($_POST['param']) && $_POST['param'] != '' && $_POST['param'] != '0'

is identical to

!empty($_POST['param'])

回答6:

isset() is not an effective way to validate text inputs and text boxes from a HTML form

You can rewrite that as "isset() is not a way to validate input." To validate input, use PHP's filter extension. filter_has_var() will tell you whether the variable exists while filter_input() will actually filter and/or sanitize the input.

Note that you don't have to use filter_has_var() prior to filter_input() and if you ask for a variable that is not set, filter_input() will simply return null.

回答7:

isset is used to determine if an instance of something exists that is, if a variable has been instantiated... it is not concerned with the value of the parameter...

Pascal MARTIN... +1 ...

empty() does not generate a warning if the variable does not exist... therefore, isset() is preferred when testing for the existence of a variable when you intend to modify it...

回答8:

When and how to use:

isset()

True for 0, 1, empty string, a string containing a value, true, false

False for null

e.g

$status = 0
if (isset($status)) // True
$status = null 
if (isset($status)) // False

Empty

False for 1, a string containing a value, true

True for null, empty string, 0, false e.g

$status = 0
if(empty($status)) // true
$status = 1
if(empty($status)) // False

回答9:

Using empty is enough:

if(!empty($variable)){
    // Do stuff
}

Additionally, if you want an integer value it might also be worth checking that intval($variable) !== FALSE.

回答10:

isset($variable) === (@$variable !== null)
empty($variable) === (@$variable == false)

回答11:

I use the following to avoid notices, this checks if the var it's declarated on GET or POST and with the @ prefix you can safely check if is not empty and avoid the notice if the var is not set:

if( isset($_GET['var']) && @$_GET['var']!='' ){
    //Is not empty, do something
}

回答12:

isset() is used to check if the variable is set with the value or not and Empty() is used to check if a given variable is empty or not.

isset() returns true when the variable is not null whereas Empty() returns true if the variable is an empty string.

回答13:

    $var = '';
// Evaluates to true because $var is empty
if ( empty($var) ) {
echo '$var is either 0, empty, or not set at all';
}
// Evaluates as true because $var is set
if ( isset($var) ) {
 echo '$var is set even though it is empty';
    }

Source: Php.net

回答14:

isset() tests if a variable is set and not null:

http://us.php.net/manual/en/function.isset.php

empty() can return true when the variable is set to certain values:

http://us.php.net/manual/en/function.empty.php

<?php

$the_var = 0;

if (isset($the_var)) {
  echo "set";
} else {
  echo "not set";
}

echo "\n";

if (empty($the_var)) {
  echo "empty";
} else {
  echo "not empty";
}
?>

回答15:

isset() vs empty() vs is_null()

来源：https://stackoverflow.com/questions/1219542/in-where-shall-i-use-isset-and-empty

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