... a general-purpose client librarynamed libmysql.
The mysqlnd library is highly optimized for and tightly integrated into PHP. The MySQL Client Library cannot offer the same optimizations because it is a general-purpose client library.
The mysqlnd library is using PHP internal C infrastructure for seamless integration into PHP. In addition, it is using PHP memory management, PHP Streams (I/O abstraction) and PHP string handling routines. The use of PHP memory management by mysqlnd allows, for example, memory savings by using read-only variables (copy on write) and makes mysqlnd apply to PHP memory limits.