PHP/CSS Random Background

Trying to get a random background between two images.

The PHP (Before Doctype in index.php):

<?php
$bg = array('back1.jpg', 'back2.jpg' ); // array of filenames
$i = rand(0, count($bg)-1); // generate random number size of the array
$selectedBg = "$bg[$i]"; // set variable equal to which random filename was chosen
?>

CSS:

body {
  height: 100%;
  margin: 0;
  color: #474747;
  font: 13px/23px  'Exo 2', sans-serif;
  min-width: 1186px;
  background: url( ../img/<?php echo $selectedBg; ?>) no-repeat 50% 50%;
  background-size: cover;
  background-attachment: fixed;
}

The issue is that the background isn't set to either of the images!