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.
Return Values
An error code value for the last call to mysqli_connect(), if it failed.
zero means no error occurred.