You can adjust following settings of PHP in Plesk 10.
PHP handler type:
ISAPI (only on Windows): This mode uses less server resources, but a website is isolated only if it is operating in a dedicated IIS application pool.
Apache module (only on Unix): This mode uses less server resources, but it is the least secure option. A site is isolated only if PHP safe mode is switched on.
FastCGI application: This mode is faster than CGI, but it consumes more memory.
Provides site isolation:
CGI application: This is the slowest mode of operation, but it uses the least amount of memory.
PHP safe mode:
Switching on PHP safe mode isolates sites, but some applications might not work because of it.
Switching off PHP safe mode is recommended to ensure stable work of most applications, but site isolation will not be provided if Apache module is selected as the PHP handler type.
PHP version (only on Windows):
Version 4.x is out-dated; you should use it only if you need to host some old PHP application versions.
Version 5.x is current and its use is recommended.
Note: When dedicated IIS application pool on Windows is enabled for a site, site isolation means that a site cannot interfere with other sites and other sites cannot interfere with it. In all other cases, site isolation implies only that a site cannot interfere with others, but other sites that are not isolated can interfere with it.