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How do you retrieve data submitted through an HTML form using PHP, and what is the difference between the GET and POST methods? What is the purpose of validating user submitted form data, and what basic validation checks are commonly performed? How does PHP's filter_var function help validate and sanitize different types of user input, such as emails, URLs, and integers? How do modern PHP frameworks like Laravel simplify form validation compared to writing manual validation checks by hand? How does Cross-Site Request Forgery protection work for form submissions, and why is including a CSRF token in every form considered essential? How should a complex, multi step form, such as a checkout process, handle validation and state management across multiple pages while maintaining data integrity and security?
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Why is server side validation always necessary even when client side JavaScript validation is already in place on a form?

Intermediate

Answer

Client side validation, while valuable for providing immediate feedback to a legitimate user without requiring a full page reload, can always be bypassed by a malicious user who disables JavaScript, directly submits a crafted request to your server, or simply modifies the request using developer tools, meaning server side validation is the only validation your application can genuinely trust, and client side validation should always be treated purely as a convenience layer rather than an actual security or data integrity control.
// Server side validation always runs regardless of client side checks
if (strlen($_POST['username']) < 3) {
    throw new ValidationException('Username too short');
}
Real-world example A security review discovers that a form relied entirely on JavaScript validation to prevent excessively long input, and an attacker easily bypassed it by submitting a request directly, prompting the team to add proper server side validation as the actual enforced rule.

Common follow-ups: What tools can an attacker use to bypass client side validation entirely?;How do modern frameworks make server side validation easier to implement consistently?

Security;Basics & Types
All 7 Form Handling & Validation questions