How can i use PhP to show hebrew letters on my website?

The display of your data is a client-side thing. If you mean modifying the HTTP header to send the charset, then you're probably looking for something like header('Content-Type: text/html; charset=utf-8'); (in this case for utf-8, which should do Hebrew just fine, AFAIK).

It's likely that just configuring your web server to send text and/or HTML files in utf-8 should be enough. With Apache 2, you'll probably want to learn about AddDefaultCharset (eg AddDefaultCharset utf-8).