( ! ) Warning: session_start(): Cannot start session when headers already sent in /var/www/includes/start_session.php on line 2 code example

Example: session_start(): cannot start session when headers already sent

// FIRST line of each php file using session (no character before)
<?php if ( empty(session_id()) ) session_start(); ?>
// Encode in ANSI or UTF8 (No BOM !)