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I'm trying to create a randomized string in PHP, and I get absolutely no output with this:

<?php
function RandomString()
{
 $characters = ’0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ’;
 $randstring = '';
 for ($i = 0; $i < 10; $i++) {
 $randstring = $characters[rand(0, strlen($characters))];
 }
 return $randstring;
}
RandomString();
echo $randstring;
?>

What am I doing wrong?