How to connect to database?

#Im getting: Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /var/www/html/test.php:2 Stack trace: #0 {main} thrown in /var/www/html/test.php on line 2

```
<?php
$link = mysql_connect('localhost', 'root', 'pass');
if (!$link) {
die('Could not connect: ' . mysql_error());
}
echo 'Connected successfully';
mysql_close($link);
?>
```