Redirect few percentage of visitors ?

Hello
I would like to know how can I redirect every 4th visitor to another website of mine.
Suppose I get 100 visits to my website(Eg: mywebsite.com)

<?php
if(some condition){
header(Location: "http://mywebsite.com/link1.php");
else{
header(Location: "http://mywebsite.com/link2.php");
}
?>

PHP:

I would like to redirect 10% to one link and rest to another link.
Can this be done ?

Please help

 

ok let say we have 100% here

we generate random number from 1 to 100. mt_rand(1,100);

so you will put the condition

if(mt_rand(1,100) <= 10) {
//goto url that in 10 range
}
else {
//goto url in 90 range
}