How can PHP developers optimize their code for handling Unicode characters in string manipulation operations?

PHP developers can optimize their code for handling Unicode characters in string manipulation operations by using the `mb_` functions provided by the Multibyte String extension. This extension allows developers to work with multibyte character encodings, such as UTF-8, which is commonly used for Unicode text. By using `mb_strlen`, `mb_substr`, and other `mb_` functions instead of their single-byte counterparts, developers can ensure that their code correctly handles Unicode characters without causing encoding issues.

// Example code snippet using mb_strlen to handle Unicode characters
$string = "Hello, 世界";
$length = mb_strlen($string, 'UTF-8');
echo "String length: " . $length;