Plesk does not provide Zend Guard Loader pre-installed. However, it can be installed manually, i.e. on Plesk 12.5, Debian-based OS, PHP 5.4:
- Download «ZendGuardLoader» according to PHP version
wget http://downloads.zend.com/guard/7.0.0/zend-loader-php5.5-linux-x86_64_update1.tar.gz 
- Extract ZendGuardLoader.solibrary and move it to modules foldercd /tmp tar -xzf zend-loader-php5.5-linux-x86_64_update1.tar.gz mv /tmp/zend-loader-php5.5-linux-x86_64/ZendGuardLoader.so /opt/plesk/php/5.5/lib64/php/modules/
- Set correct permissions
chmod 755 /opt/plesk/php/5.5/lib64/php/modules/ZendGuardLoader.so chown root:root /opt/plesk/php/5.5/lib64/php/modules/ZendGuardLoader.so
- Create corresponding .inifile with the following content:nano /opt/plesk/php/5.5/etc/php.d/zendGuardLoader.iniAdd text: 
- 
zend_extension=ZendGuardLoader.so 
- Restart php-fpmservice:service plesk-php55-fpm restart