PHP fastest way to iterate over list? [duplicate]

This question already has an answer here:

• Testing multiple values efficiently in PHP 3 answers

I need to iterate over 15 cases, about 2 mio times per day. This cases are my "blocked ISPs list". If a visitor has this ISP, they will not see my site. When a visitor visits my site, its ISP will be detected and saved under $ISP. This is my current solution:

<?php
$ISP = "vodafone";

switch ($ISP) {
    case "telecom":
        x=1;
        break;
    case "vodafone":
        x=1;
        break;

...

  default:
        x=0;
    }
    ?>