Check whether $_POST-value is empty

If the form was successfully submitted, $_POST['userName'] should always be set, though it may contain an empty string, which is different from not being set at all. Instead check if it is empty()

if (isset($_POST['submit'])) {
    if (empty($_POST['userName'])) {
        $username = 'Anonymous';
    } else { 
        $username = $_POST['userName'];
    }
}

isset() will return true if the variable has been initialised. If you have a form field with its name value set to userName, when that form is submitted the value will always be "set", although there may not be any data in it.

Instead, trim() the string and test its length

if("" == trim($_POST['userName'])){
    $username = 'Anonymous';
}      

To check if the property is present, irrespective of the value, use:

if (array_key_exists('userName', $_POST)) {}

To check if the property is set (property is present and value is not null or false), use:

if (isset($_POST['userName'])) {}

To check if the property is set and not empty (not an empty string, 0 (integer), 0.0 (float), '0' (string), null, false or [] (empty array)), use:

if (!empty($_POST['userName'])) {}

Question: Check whether a $_POST value is empty.

Translation: Check to see if an array key/index has a value associated with it.

Answer: Depends on your emphasis on security. Depends on what is allowed as valid input.

1. Some people say use empty().

From the PHP Manual:

"[Empty] determines whether a variable is considered to be empty. A variable is considered empty if it does not exist or if its value equals FALSE."

The following are thus considered empty.

"" (an empty string)
0 (0 as an integer)
0.0 (0 as a float)
"0" (0 as a string)
NULL
FALSE
array() (an empty array)
$var; (a variable declared, but without a value)

If none of these values are valid for your input control, then empty() would work. The problem here is that empty() might be too broad to be used consistently (the same way, for the same reason, on different input control submissions to $_POST or $_GET). A good use of empty() is to check if an entire array is empty (has no elements).

2. Some people say use isset().

isset() (a language construct) cannot operate on entire arrays, as in isset($myArray). It can only operate on variables and array elements (via the index/key): isset($var) and isset($_POST['username']). The isset()language construct does two things. First it checks to see if a variable or array index/key has a value associated with it. Second, it checks to make sure that value is not equal to the PHP NULL value.

In short, the most accurate check can be accomplished best with isset(), as some input controls do not even register with $_POST when they are not selected or checked. I have never known a form that submitted the PHP NULL value. None of mine do, so I use isset() to check if a $_POST key has no value associated with it (and that is not NULL). isset()is a much stricter test of emptiness (in the sense of your question) than empty().

3. Some people say just do if($var), if($myArray), or if($myArray['userName']) to determine emptiness.

You can test anything that evaluates to true or false in an if statement. Empty arrays evaluate to false and so do variables that are not set. Variables that contain the PHP NULL value also evaluate to false. Unfortunately in this case, like with empty(), many more things also evaluate to false: 1. the empty string '', zero (0), zero.zero (0.0), the string zero '0', boolean false, and certain empty XML objects.

--Doyle, Beginning PHP 5.3

In conclusion, use isset() and consider combining it with other tests. Example:

May not work due to superglobal screwiness, but would work for other arrays without question.

if (is_array($_POST) && !empty($_POST)) {
    // Now test for your successful controls in $_POST with isset()
} 

Hence, why look for a value associated with a key before you even know for sure that $_POST represents an array and has any values stored in it at all (something many people fail to consider)? Remember, people can send data to your form without using a web browser. You may one day get to the point of testing that $_POST only has the allowed keys, but that conversation is for another day.

Useful reference:

PHP Manual: Type Testing Comparison Tables