PHP open_basedir errors occur when a script tries to access a directory outside of httpdocs. For security reasons Plesk only allows access to the httpdocs and tmp directories.

So what if you need to access a different directory?

The easiest way is to amend the vhost.conf file of the
specific domain.

Open the vhost.conf file if it exists. If not...create it. The vhost.conf file is found under /PATH-TO-DOMAIN/YourDomain.tld/conf/vhost.conf

Add these lines:-

<Directory /PATH-TO-DOMAIN/YourDomain.tld/httpdocs>
php_admin_value open_basedir /PATH-TO-DOMAIN/YourDomain.tld/httpdocs:/tmp:/ADD-YOUR-DIR:/
</Directory>

Save the vhost.conf file and run this command to load the changes into Apache

/usr/local/psa/admin/sbin/websrvmng -a -v

Restart Apache....your finished