How to write an Irule to delete session cookies so that we can enhnace security level.

Do you wish the cookies to be invalidated in client browser after the logout request?

Should get you started:

when HTTP_REQUEST {
  set logOut 0
  set cookiesInRequest [HTTP::cookie names]
  if {[string tolower [HTTP::path]] ends_with '/logout.php'}{
    set logOut 1
  }
}

when HTTP_RESPONSE {
  if { $logOut == 1 }{
    foreach aCookie $cookiesInRequest {
      HTTP::cookie expires $aCookie 1 absolute
    }
  }
}