How can PHP developers ensure that only the source code is affected when converting special characters to HTML codes?

When converting special characters to HTML codes in PHP, developers can ensure that only the source code is affected by using the htmlspecialchars function. This function converts special characters like <, >, ", ', and & to their corresponding HTML entities, preventing any potential security vulnerabilities such as cross-site scripting (XSS) attacks.

$string = "<script>alert('Hello, world!');</script>";
$encoded_string = htmlspecialchars($string, ENT_QUOTES, 'UTF-8');
echo $encoded_string;