Any PHP extension that is linked against the PDO core module can elect to register a PDO driver. A PDO driver is represented by the pdo_driver_t type which describes the version of PDO against which the driver was built, the name of the driver and a factory method that can be used to instantiate a database connection handle. A connection handle is represented by the pdo_dbh_t type which describes, among other things, how to instantiate a prepared statement handle, represented by the pdo_stmt_t type. These three types are the main interface between the PDO core and a PDO driver. In this chapter, we'll refer to an imaginary skeleton driver, and use SKEL as a placeholder for its various functions or types. In practice, a given database vendor will typically choose to use a short, lowercase, version of the name of their database or client library in place of SKEL.

A driver extension will typically statically define an instance of pdo_driver_t and pass the address of it to php_pdo_register_driver during its module initialization callback (also known as MINIT, for module init), and again to php_pdo_unregister_driver during its module shutdown callback (also known as MSHUTDOWN). php_pdo_driver.h defines pdo_driver_t, php_pdo_register_driver and php_pdo_unregister_driver as follows: is_persistent and pass that flag to * pemalloc() for all allocations that are stored in the dbh or your instance * data in the db, otherwise you will crash PHP when persistent connections * are used. */ int (*db_handle_factory)(pdo_dbh_t *dbh, zval *driver_options TSRMLS_DC); } pdo_driver_t; /* call this in MINIT to register your PDO driver */ PDO_API int php_pdo_register_driver(pdo_driver_t *driver); /* call this in MSHUTDOWN to unregister your PDO driver */ PDO_API void php_pdo_unregister_driver(pdo_driver_t *driver); ]]> A driver would typically use code like the following to register a driver: The SKEL_db_handle_factory is provided by the driver; its operation will be discussed in the next section.