Returns the last error code number from the last call to mysqli_connect().
Note: Client error message numbers are listed in the MySQL errmsg.h header file, server error message numbers are listed in mysqld_error.h. In the MySQL source distribution you can find a complete list of error messages and error numbers in the file Docs/mysqld_error.txt.
An error code value for the last call to mysqli_connect(), if it failed. zero means no error occurred.