Input/Output Learning Mode

Consider the following script. What will the file myfile.txt contain at the end of its execution?

<?php
$array = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$f = fopen ("myfile.txt", "r");
for ($i = 0; $i < 50; $i++) {
fwrite ($f, $array[rand(0, strlen ($array) - 1)]);
}
?>