PHP Error: Function name must be a string

Cause:

<?php
if (!isset($_session)) {
    $session_start();
}
?>

The issue here is that session_start() is a function, not a variable. Adding a $ makes PHP treat it like a variable function call, which is invalid.

Fix:

1. Use session_start() directly without $.

2. Use the correct superglobal $_SESSION (all caps).

Correct Code:

<?php
if (!isset($_SESSION)) {
    session_start();
}
?>