<?php

DEFINE('DBNAME','mydbname');

DEFINE('DBUSER','root');

DEFINE('DBPWD','password');

DEFINE('DBHOST','localhost');

?>

<?php

$DBNAME = 'mydbname';
$DBUSER = 'root';
$DBPWD = 'password';
$DBHOST = 'localhost';

?>

<?php

$dbh = new PDO('mysql:dbname='.DBNAME.';host='.DBHOST, DBUSER, DBPWD);
…
?>

echo 'Database name:' . DBNAME;

//Procedural style
$conn = mysqli_connect(DBHOST, DBUSER, DBPWD);
mysqli_select_db($conn, DBNAME);

//Object Oriented style
$conn = new mysqli(DBHOST, DBUSER, DBPWD);
$conn->select_db(DBNAME);

$conn = new PDO('mysql:host=' . DBHOST . ';dbname=' . DBNAME, DBUSER, DB_PWD);