$db_host,$db_username,$db_password,$db_database is variable and the below mysqli_connect is a function. $connection is also a variable. i am describing about this connection. You will change a variable name easily its not a necessary you are using a $connection or $conn or something like that. Now came on function ..because our hidden part is mysqli_connect function.Because Mostly Developer using this function. Developers know a one thing only if we using mysqli_connect then Database Connection will be established. Dnt know About the Backend of function.
that type of code are using in mysqli_connect Function
resource mysql_connect ([ string
$server = ini_get(“mysql.default_host”) [, string
$username = ini_get(“mysql.default_user”) [, string
$password = ini_get(“mysql.default_password”) [, bool
FALSE [, int
$client_flags = 0 ]]]]] )
For Php 5.5 version We are using mysql_connect.
For Php 7 version We are using mysqli_connect or PDO.
echo “Sql Error”;