您要使用正则表达式. \b与单词边界匹配.

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

$text = preg_replace('/\bHello\b/', 'NEW', $text);

如果$text包含UTF-8文本，则必须添加Unicode修饰符"u"，以便不会将非拉丁字符误解为单词边界:

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('/\bHello\b/u', 'NEW', $text);