A script may include many files, this enables PHP coders to write some of the code once, and use it in many scripts, to include an external file into your script, you can use one of the following:
1. include: reads the external file, and interprets it, if the external file can’t be found, a warning is produced, and the execution continues.
2. require: the same as includes, but it causes the execution to stop.
3. include_once: works the same way as include, but the included file will be interpreted just once, at the first time it’s included, if it’s included again in the same file, it won’t be interpreted again.
4. require_once: the same as require, but also interpreted once like include_once.