For Plesk version 11
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Select "PHP Settings" from "Websites & Domains" tab.
4. Select "on" from "safe_mode" drop down menu.
5. Click on "OK" button.
For Plesk version 9
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Select "Web Hosting Settings" from "Website" tab.
4. Enable the check box for "PHP safe_mode" option.
5. Click on "OK" button.