What is Input Validation?

Input validation is the first line of defense against injection attacks, data corruption, and application errors. Validation should happen on both client-side (for user experience) and server-side (for security — client-side validation can be bypassed). Approaches include allowlisting (accepting only known-good patterns), denylisting (rejecting known-bad patterns — less secure), type checking, length limits, range validation, and format validation (regex for emails, dates). In PHP, filter_var() and filter_input() provide built-in validation. Never trust user input — validate everything from form fields to HTTP headers, cookies, file uploads, and API parameters. Validation failures should return clear error messages without revealing system internals.