isset() Function in PHP: Checking Variable Existence and Value

The isset() function in PHP is used to determine if a variable is set and is not NULL. It returns a boolean value (true or false) based on whether the variable exists and has a value. This function is commonly used for tasks such as checking if a form input has been submitted or if a variable has been initialized before using it in a code block.

Example Usage:

$name = 'John';
if (isset($name)) {
    echo 'Hello, ' . $name;
} else {
    echo 'Name is not set.';
}

In this example, the isset() function checks if the $name variable exists and has a value. If it does, the code block will output 'Hello, John'. If not, it will output 'Name is not set.'