Exploit:PHP/Preamble is a detection for a family of various PHP scripts. The scripts are used to test whether a particular site is vulnerable for a Remote File Inclusion (RFI) exploitation.
Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.
More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.
You may also refer to the Knowledge Base on the F-Secure Community site for more information.
The scripts belonging to this family are usually used to test whether a site or a webpage is vulnerable to a Remote File Inclusion (RFI) vulnerability.
The scripts test various methods an attacker can use to introduce a malicious backdoor script onto the affected server.