How can PHP developers ensure code compatibility when upgrading from older versions like PHP 5.3.6 to newer versions like PHP 5.4 or above, especially in terms of deprecated functions and syntax changes?

When upgrading from older versions like PHP 5.3.6 to newer versions like PHP 5.4 or above, PHP developers can ensure code compatibility by identifying and replacing deprecated functions and updating syntax changes. This can be achieved by referring to the official PHP documentation for deprecated features and changes in syntax, and making the necessary modifications in the codebase.

// Example code snippet showing how to replace a deprecated function
// Deprecated function in PHP 5.3.6
$oldVar = mysql_real_escape_string($input);

// Updated code for PHP 5.4 and above
$newVar = mysqli_real_escape_string($connection, $input);