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What are the best practices for handling form validation in PHP to prevent errors like headers already sent?

When handling form validation in PHP, it's important to ensure that no output is sent to the browser before sending headers. To prevent errors like "headers already sent," you should perform form validation before any output is generated, such as HTML or whitespace characters. One common practice is to use PHP's output buffering functions to buffer the output until all validation checks are complete.

php 
<?php
ob_start(); // Start output buffering

// Perform form validation here

if ($form_valid) {
    // Process form data
    header("Location: success.php");
    exit();
} else {
    // Display error messages
}

ob_end_flush(); // Flush the output buffer
?>

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