How does filter_has_var differ from isset in PHP, and when should each be used?

filter_has_var is a function in PHP that checks if a variable of a specified input type exists. It is typically used to check if a variable sent via GET or POST method exists. On the other hand, isset is a language construct in PHP that checks if a variable is set and is not NULL. It is used to determine if a variable has been initialized with a value. When working with form inputs or variables sent via GET or POST methods, it is recommended to use filter_has_var to specifically check for those variables. On the other hand, isset should be used when checking for the existence of variables in general, regardless of their input source.

// Using filter_has_var
if (filter_has_var(INPUT_POST, 'username')) {
    $username = $_POST['username'];
    // Process the username
}

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