Where is data filtering performed?

Data filtering can be performed at multiple levels:

1. On the client side (browser):

   • JavaScript is used for interactive filtering without reloading the page.
   • Suitable for small data sets or for improving UX.
   • Does not ensure security and does not reduce server load.

2. On the server side (PHP):

   • The main place for filtering data before outputting it to the client.
   • Uses built-in PHP functions, regular expressions, data validation.
   • Ensures data security and correctness, reduces the volume of data transmitted.

3. At the database level:

   • The most efficient way to filter large volumes of data.
   • Uses SQL queries with WHERE, HAVING conditions.
   • Optimized for searching and data retrieval.

Example of server-side filtering (PHP):

<?php

// Getting data from user input
$userInput = $_POST['data'] ?? '';

// Stripping HTML tags and potentially harmful content
$filteredData = htmlspecialchars(strip_tags($userInput), ENT_QUOTES, 'UTF-8');

// Example of filtering by a specific pattern (e.g., digits only)
if (preg_match('/^\d+$/', $filteredData)) {
    // Data passed validation
    echo "Valid data: " . $filteredData;
} else {
    // Data does not match the pattern
    echo "Invalid data.";
}

?>