An easy to include function for connecting to an SQL database. Return the database handle. Fill in your default connection, while allowing for case-by-case connection overrides.

db_connect.php

<?php

function db_connect ($options = []) {
  if (!empty($options)) {
    $hostName     = !empty($options['hostName'])     ? $options['hostName']     : 'someHostName';
    $databaseName = !empty($options['databaseName']) ? $options['databaseName'] : 'someDatabaseName';
    $dbUserName   = !empty($options['dbUserName'])   ? $options['dbUserName']   : 'someUsername';
    $dbPassword   = !empty($options['dbPassword'])   ? $options['dbPassword']   : 'somepassword';
  } else {
    $hostName     = 'someHostName';
    $databaseName = 'someDatabaseName';
    $dbUserName   = 'someUsername';
    $dbPassword   = 'somepassword';
  }

  $mysqli = new mysqli($hostName, $dbUserName, $dbPassword, $databaseName);
  // Check for errors
  if (mysqli_connect_errno()) {
    exit(mysqli_connect_error());
  }
  if (!$mysqli->set_charset('utf8')) {
    exit("Error loading character set utf8 for db $databaseName: %s\n".$mysqli->error);
  }

  return $mysqli;
}