What are the common pitfalls when upgrading PHP from version 5.2 to a newer version like 5.4 or 5.6?

One common pitfall when upgrading PHP from version 5.2 to a newer version like 5.4 or 5.6 is deprecated functions or features that are no longer supported. To solve this issue, you should update your code to use the recommended alternative functions or features.

// Deprecated function in PHP 5.2
mysql_connect($server, $username, $password);

// Updated function in PHP 5.4 or higher
mysqli_connect($server, $username, $password);