What are the common issues faced when upgrading to PHP5 from a lower version in terms of MySQL functions?

When upgrading to PHP5 from a lower version, one common issue faced is the deprecation of the MySQL extension in favor of MySQLi or PDO. To solve this issue, you need to update your code to use MySQLi or PDO functions for interacting with MySQL databases.

// Before upgrading to PHP5
$conn = mysql_connect($host, $username, $password);
mysql_select_db($database, $conn);
$result = mysql_query($query, $conn);

// After upgrading to PHP5
$conn = mysqli_connect($host, $username, $password, $database);
$result = mysqli_query($conn, $query);