PHP Variable Handling isset() Function

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The PHP Variable Handling isset() function is used to check that a variable is set or null. It means that the variable needs to be declared and assigned a value other than null. If a variable is deleted using unset(), isset() returns false. If a variable is null, isset() returns false.

In PHP, a null character ("\0") does not mean null. If you check many variables, isset() returns true if they are all set. It checks the variables in order from left to right and stops if any of them are not set. This function helps to prevent errors when using variables.

Syntax

Below is the syntax of the PHP Variable Handling isset() function −

bool isset ( mixed $var, mixed ...$vars )

Parameters

Below are the parameters of the isset() function −

• $var − It is the variable that needs to be checked.

• $vars − It is more variables that can also be checked.

Return Value

The isset() function returns TRUE if the variable exists and is not null. And it returns FALSE if the variable does not exist or is null.

PHP Version

First introduced in core PHP 4, the isset() function continues to function easily in PHP 5, PHP 7, and PHP 8.

Example 1

Here is the basic example of the PHP Variable Handling isset() function to check if a single variable is set or not. It returns true if the variable has a value, otherwise false.

<?php
   // Declare a variable with a value
   $var = "Hello, PHP!"; 

   if (isset($var)) {
      echo "Variable is set.";
   } else {
      echo "Variable is not set.";
   }
?>

Output

Here is the outcome of the following code −

Variable is set.

Example 2

In the below PHP code we will use the isset() function and check an unset variable. So the program first declares a variable and then removes it with the help of the unset(). So the function will return false.

<?php
   // Declare a variable
   $var = "Hello, World!"; 
  
   // Remove the variable
   unset($var); 

   if (isset($var)) {
      echo "Variable is set.";
   } else {
      echo "Variable is not set.";
   }

?> 

Output

This will generate the below output −

Variable is not set.

Example 3

Now the below code checks multiple variables and check if they are set using the isset() function. So the program checks multiple variables and it returns true only if all variables are set and not null.

<?php
   $var1 = "PHP"; 
   $var2 = "is awesome"; 
   // This variable is null
   $var3 = null; 

   if (isset($var1, $var2, $var3)) {
      echo "All variables are set.";
   } else {
      echo "At least one variable is not set.";
   }
?> 

Output

This will create the below output −

At least one variable is not set.

Example 4

In the following example, we are using the isset() function with an array. So the program will check if a specific key exists in an array. And it prevents errors when accessing undefined keys.

<?php
   // Declare an array
   $user = ["name" => "Sourabh", "age" => 30]; 

   if (isset($user["name"])) {
      echo "Name exists: " . $user["name"];
   } else {
      echo "Name does not exist.";
   }

?> 

Output

Following is the output of the above code −

Name exists: Sourabh