What are the potential issues when migrating from PHP4 to PHP5, especially in terms of variable handling like register_globals?

When migrating from PHP4 to PHP5, one potential issue is the change in variable handling, specifically the removal of the `register_globals` feature. This means that variables are no longer automatically registered as global variables, and must be accessed using `$_GET`, `$_POST`, `$_COOKIE`, etc. To solve this issue, you can update your code to explicitly access variables from the appropriate superglobal arrays.

// PHP4 style variable handling
$my_var = $_GET['my_var'];

// PHP5 style variable handling
$my_var = isset($_GET['my_var']) ? $_GET['my_var'] : null;