What potential issues can arise when upgrading from PHP 5.6 to PHP 7.3?

One potential issue when upgrading from PHP 5.6 to PHP 7.3 is that certain functions or features deprecated in PHP 7.0 may no longer work. To solve this, you will need to update your code to use the recommended alternatives or refactor the deprecated functions. Additionally, any syntax errors or compatibility issues between PHP 5.6 and PHP 7.3 should be addressed during the upgrade process.

// Example of updating code to use the recommended alternative for a deprecated function
// Deprecated in PHP 7.0: count(): Parameter must be an array or an object that implements Countable
// Updated code for PHP 7.3:
if (is_countable($array)) {
    $count = count($array);
} else {
    $count = 0;
}