如何防止在包含的文件中包含两次相同的文件?

In a main index.php file, header.php is included. Both files must insert session.php, database.php etc. to run. But when header.php is included, there are two same require/include files, so there are two same functions, queries. This cause some problems in the file.
What is the solution of this problem?

解决方案

Use include_once instead of include.

The include_once statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include statement, with the only difference being that if the code from a file has already been included, it will not be included again. As the name suggests, it will be included just once.

http://php.net/manual/en/function.include-once.php