<?php
$red = rand(0,255);
$green = rand(0, 255);
$blue = rand(0, 255);
?>
<style>
#random {
    width:100px;
    height:100px;
    background-color: rgb( <?php echo join(',', [$red, $green, $blue]) ?> )
}
</style>
<div id="random">

</div>