Basically, MySQL is the old database driver, and MySQLi is the Improved driver. The "i" stands for "improved" so it is MySQL improved.MySQLi can be done procedural and object-oriented whereas MySQL can only be used procedurally. Mysqli also supports prepared statements which protect from SQL Injection.The main useful features are:an Object-oriented interface support for prepared statements support for multiple statements support for transactions enhanced debugging capabilities embedded server support. NOTE: MySQL was deprecated in PHP 5.5.0 and was removed in PHP 7.

mysqli is database drive which is used for php languages and provides an interface for mysql, there are 3 main API option for connecting mysql database PHP's mysql extension, PHP's mysqli extension, PHP Data Objects(PDO). I in mysql are Improved mysql