PHP Form Validation and Sanitization Guide

To validate and sanitize a PHP form safely: read each field from $_POST with a default, trim() it, check it against clear rules with functions like filter_var(), mb_strlen() and an allow-list, collect the errors in an array, and only use the data when there are none. Then escape every value with htmlspecialchars() when you output it into HTML, and use prepared statements when you store it. In this guide I build a complete contact form handler step by step on PHP 8.4, including CSRF protection and the Post/Redirect/Get pattern. When I started with PHP, I thought "sanitizing" meant running every