Real-World PHP Security

PHP's Safe mode is something you should learn to work with whether you are a PHP developer or a system administrator. Safe mode is a set of configuration options that allow the system administrator to alter the behavior of the PHP interpreter by implementing security measures. From a system administrator's point of view, this means you must learn how to implement this feature properly, without making it impossible for developers to set up their applications on your server. From a developer's point of view, you must learn what possibly could get broken in your application if this feature is turned on.

Turning safe_mode on makes sense if you manage a shared server that serves PHP applications and the PHP developers using this server are not trusted. Enabling safe_mode in your php.ini file effectively makes any file-related operation in any of your scripts impossible unless the UID of the owner of the file is the same as the UID of the running script. PHP also gives you the ability to change this policy while safe_mode is on by turning on the safe_mode_gid option. In this case, PHP checks for the GID of the files you are trying to work with instead of their UID.

It also is good practice to not let your users execute any system binary they want; safe_mode_exec_dir comes into play here. This priceless feature lets you tell PHP not to perform any binary execution, through exec() or any other function, unless the binary is located in the safe_mode_exec_dir, such as /usr/local/php/bin.

Once you have familiarized yourself with the restrictions implemented by PHP when safe_mode is enabled, you should be able to develop software that doesn't break when it's run on servers with this directive enabled. Many ISPs use safe_mode. The simple guidelines to follow are:

System administrators also have at their disposal other powerful tools to ensure the overall security of their systems. These tools include disable_functions that prevent specified functions from being called, as well as options such as open_basedir, which limit any file operation to a specific directory.

The PHP documentation team has provided an extensive amount of literature on the subject. They also have provided documentation for every aspect of safe_mode and related functions and directives.