isset() Function in PHP: Check if a Variable is Set

isset() is a built-in PHP function that checks if a variable is set and not null. It returns a boolean value (true or false) depending on whether the variable is set or not. This function is commonly used to prevent undefined variable errors in PHP code. Here's an example:

$name = 'John';
if (isset($name)) {
   echo 'The variable is set.';
} else {
   echo 'The variable is not set.';
}

In the above example, the isset() function checks if the variable $name is set or not. Since it has been assigned a value ('John'), the function returns true and the message 'The variable is set.' is displayed.