You want to use regular expressions. The \b
matches a word boundary.
$text = preg_replace('/\bHello\b/', 'NEW', $text);
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);