What is the crucial difference between include and require in PHP when a target file is missing?
Option A is correct. include emits a warning E_WARNING if the file cannot be loaded, but execution continues. require raises a fatal E_COMPILE_ERROR and halts script execution immediately.
Which PHP function must be called at the very top of a web script before any HTML output to initialize or resume session state?
Option D is correct. session_start() creates a new session or resumes an existing one based on a session identifier passed via a GET/POST request or cookie. It must precede any output.
Which function in PHP is used to prevent Cross-Site Scripting (XSS) by escaping HTML special characters in user input?
Option B is correct. htmlspecialchars() converts special characters like <, >, &, ', " to HTML entities (e.g., <), preventing user-submitted HTML/JS scripts from executing in browsers.
What is the default file extension for PHP scripts executed on a web server?
Option B is correct. Standard PHP files have a .php file extension. The web server engine (like Apache or Nginx with PHP-FPM) parses code inside <?php ... ?> tags in .php files.
In PHP, which operator is used for string concatenation?
Option C is correct. PHP uses the dot . operator for string concatenation (e.g., $full = $fname . ' ' . $lname;). In JavaScript and Python, + is used.