PHP 函数如何返回字符串？

function getGreeting() {
  return "Hello, world!";
}

echo getGreeting(); // 输出 "Hello, world!"

$greeting = "Greetings from PHP!";

function returnGreeting() {
  return $greeting;
}

echo returnGreeting(); // 输出 "Greetings from PHP!"

function buildMessage($name, $age) {
  return "Hello, $name! You are $age years old.";
}

echo buildMessage("John", 30); // 输出 "Hello, John! You are 30 years old."

function getAcronym($words) {
  $acronym = "";
  foreach (explode(" ", $words) as $word) {
    $acronym .= strtoupper($word[0]);
  }
  return $acronym;
}

echo getAcronym("United States of America"); // 输出 "USA"