The isset() function is used to check whether a variable is set or not. If the variable is already unset with the unset() function, it will no longer be set.

PHP isset

PHP isset() is a built-in function that checks whether a variable is set and is not NULL. The isset() function returns false if the testing variable contains a NULL. It accepts only variable references as arguments and not direct values.

The isset()  function returns TRUE if a given variable is set to any value, including the empty string.

Syntax

See the following syntax.

bool isset($var, mixed)

Parameters

The first parameter of this function is $var. This parameter is used to store the value of the variable. The mixed indicates that a parameter may accept multiple.

Example

Let’s take a string variable example.

<?php

$stranger = 'Eleven';
var_dump(isset($stranger));

See the output.

➜  pro php app.php
bool(true)
➜  pro

Now, check the empty string.

<?php

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

See the output.

➜  pro php app.php
bool(true)
➜  pro

Let’s check the NULL value.

<?php

$stranger = NULL;
var_dump(isset($stranger));

See the output.

➜  pro php app.php
bool(false)
➜  pro

Let’s check the direct values and not the variables.

<?php

var_dump(isset('Eleven'));

See the following output.

➜  pro php app.php
PHP Fatal error:  Cannot use isset() on the result of an expression 
(you can use "null !== expression" instead) in /Users/krunal/Desktop/code/php/pro/app.php on line 3

Fatal error: Cannot use isset() on the result of an expression 
(you can use "null !== expression" instead) in /Users/krunal/Desktop/code/php/pro/app.php on line 3
➜  pro

So, I cannot use isset() on the result of an expression.

isset $_POST

Let’s say I have the form on one page that submits data to another. There, it checks if an input mail field is filled. If filled, do something; if it is not filled, do other things.

In the above scenario, we can check whether the variable is set. Based on that, we can put the server-side validation.

See the following code to implement the above scenario.

if (isset($_POST["mail"]) && !empty($_POST["mail"])) {
    echo "Yes, mail is set";    
} else {  
    echo "N0, mail is not set";
}

isset() function with multiple parameters

Let’s check more than one variable inside the isset() function.

<?php

$mage = 'Eleven';
$villian = 'Mind Flayer';

var_dump(isset($mage, $villian));

See the following output.

➜  pro php app.php
bool(true)
➜  pro

The empty() is the opposite of (boolean) var, except that no warning is generated when the variable is not set. See the following code.

if (!empty($_POST["mail"])) {
    echo "Yes, mail is set";    
} else {  
    echo "N0, mail is not set";
}

That’s it for this tutorial.