How does changing from using $submit to isset($_POST['submit']) impact the handling of form data in PHP?

Changing from using $submit to isset($_POST['submit']) ensures that the form data is properly handled in PHP. Using isset($_POST['submit']) checks if the form has been submitted and the 'submit' button has been clicked, allowing you to process the form data securely.

if(isset($_POST['submit'])) {
    // Process form data here
}