Anyone who prefers to run a locked down site may have chosen to disable user 1 (avoid risk that the password is guessed) and disable the PHP module (for example site policy is to avoid using PHP input filter).
If you are comfortable writing a php hook in your custom module, you can do this (taken from php.module):
/**
* Implements hook_permission().
*/
function XXX_permission() {
if (!module_exists('php')) {
return array(
'use PHP for settings' => array(
'title' => t('Use PHP for settings'),
'restrict access' => TRUE,
),
);
}
}
return $permissions;
}