Modify the open_basedir settings in your PHP configuration (See Runtime Configuration).
The open_basedir setting is primarily used to prevent PHP scripts for a particular user from accessing files in another user's account. So usually, any files in your own account should be readable by your own scripts.
Example settings via .htaccess if PHP runs as Apache module on a Linux system:
<DirectoryMatch "/home/sites/site81/">
php_admin_value open_basedir "/home/sites/site81/:/tmp/:/"
</DirectoryMatch>