What potential issues can arise when migrating a PHP application from PHP 4 to PHP 5, specifically related to session handling?

When migrating a PHP application from PHP 4 to PHP 5, potential issues related to session handling may arise due to changes in how sessions are managed and stored. One common issue is the deprecated use of the session_register() function in PHP 4, which should be replaced with the $_SESSION superglobal array in PHP 5.

// Replace session_register() with direct assignment to $_SESSION superglobal array
// Before migration
session_start();
$var = "value";
session_register('var');

// After migration
session_start();
$var = "value";
$_SESSION['var'] = $var;