It refers to storing data to storage devices for accessing later.
In PHP scripts, you can make information available within sessions — periods of time that users spend at your Web site — by using methods such as PHP session functions and by submitting forms.
The information needs to be stored on the server in flat files or in databases for persistence. Flat files are text files stored in the computer file system which can be accessed and edited by using any text file editor. Using a database for data storage needs an database software like MySQL or Oracle. The data is stored in files created by the database software and can only be accessed by the database software. You don't need to know how the data is stored, just how to interact with the database software.
In PHP 5, SQLite, an extension for data storage that combines the main advantages of flat files and databases, is included by default. SQLite stores the data in a flat file, so you don't need to install database software, but you store data using SQL, the standard database communication language. SQLite is a quick option for storing and retrieving small amounts of data in a flat file using SQL. SQLite is not a good option for really huge, complicated databases.