您想使用正则表达式. 匹配单词边界.

You want to use regular expressions. The matches a word boundary.

$text = preg_replace('/Hello/', 'NEW', $text);

如果 $text 包含 UTF-8 文本，则必须添加 Unicode 修饰符u"，以便非拉丁字符不会被误解为单词边界:

If $text contains UTF-8 text, you'll have to add the Unicode modifier "u", so that non-latin characters are not misinterpreted as word boundaries:

$text = preg_replace('/Hello/u', 'NEW', $text);