Input Validation

Input validation is a vital aspect of PHP security. It involves checking whether the user-provided data is in the expected format or not before it's processed further. This helps prevent potential security risks such as SQL injections, cross-site scripting (XSS) etc. Let's take an example of a simple form input validation:

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
  echo("Email is valid");
} else {
  echo("Email is not valid");
}

This code uses PHP's built-in filter_var() function to ensure the data is a valid email address. If not, the form will not be submitted until valid data is entered.