What is the purpose of validating user submitted form data, and what basic validation checks are commonly performed?

Validating form data ensures that the information submitted by a user actually meets your application's expectations before it is processed or stored, with common checks including verifying that required fields are not empty, that an email field actually contains a properly formatted email address, that a numeric field contains a genuine number within an acceptable range, and that text fields do not exceed a reasonable maximum length, all of which help prevent bad or malicious data from causing problems later in your application.

if (empty($_POST['email']) || !filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
    $errors[] = 'A valid email address is required';
}