If you want to remove all of the open_basedir restrictions for all sites in Plesk, simply create a file called /etc/httpd/conf.d/zzz_openbasedir_removal.conf and add this text within it:
<DirectoryMatch /var/www/vhosts/(.*)/httpdocs/>
php_admin_value open_basedir none
</DirectoryMatch>
Just a note, this isn't a terribly great idea from a security standpoint. 
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hi