What are common issues when transitioning a website from PHP 5.6 to PHP 7.2?

One common issue when transitioning a website from PHP 5.6 to PHP 7.2 is the removal of deprecated functions and features. This can cause errors or warnings in the code. To solve this, it is necessary to update the code to use the recommended alternatives or newer functions provided in PHP 7.2.

// Deprecated function in PHP 5.6
mysql_connect($host, $user, $password);

// Updated code for PHP 7.2
$mysqli = new mysqli($host, $user, $password);