What is the difference between isset() and empty() in PHP and when should each be used?

isset() is used to check if a variable is set and is not NULL, while empty() is used to check if a variable is empty (i.e., it does not exist, is NULL, or has a value that is considered empty like 0, an empty string, or an empty array). isset() should be used when you want to check if a variable is set, and empty() should be used when you want to check if a variable is empty. Example:

// Using isset()
if(isset($variable)){
    // $variable is set and not NULL
} else {
    // $variable is not set or is NULL
}

// Using empty()
if(empty($variable)){
    // $variable is empty
} else {
    // $variable is not empty
}