如何将字符串php中的特定单词小写

function convertWordToLowercase(str, word) {
  var words = str.split(" ");  // 将字符串分割为单词数组
  for (var i = 0; i < words.length; i++) {
    if (words[i] === word) {  // 判断是否为特定单词
      words[i] = words[i].toLowerCase();  // 将特定单词转换为小写
    }
  }
  var newStr = words.join(" ");  // 将单词数组拼接回字符串
  return newStr;
}

var originalString = "This is a PHP string.";
var specificWord = "PHP";
var newString = convertWordToLowercase(originalString, specificWord);
console.log(newString);