How can the execution of PHP code be triggered based on user interaction in a web application?

To trigger the execution of PHP code based on user interaction in a web application, you can use HTML forms with PHP scripts handling the form submissions. By creating a form with input fields and a submit button, you can collect user input and send it to a PHP script for processing. The PHP script can then perform actions based on the user's input.

// HTML form to collect user input
<form method="post" action="process.php">
  <input type="text" name="username" placeholder="Enter your username">
  <input type="submit" value="Submit">
</form>

// process.php - PHP script to handle form submission
<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $username = $_POST["username"];
  
  // Perform actions based on user input
  echo "Hello, $username!";
}
?>