What are the differences between isset() and empty() in PHP, and when is it appropriate to use each one?

isset() is used to check if a variable is set and not null, while empty() is used to check if a variable is empty (i.e., "", 0, null, false, or an empty array). Use isset() when you want to check if a variable exists and has a non-null value, and use empty() when you want to check if a variable is empty. Example:

// Using isset()
$var = "Hello";
if(isset($var)){
    echo "Variable is set and not null";
} else {
    echo "Variable is not set or is null";
}

// Using empty()
$var = "";
if(empty($var)){
    echo "Variable is empty";
} else {
    echo "Variable is not empty";
}