Add weight factor to random sorting in PHP

<?php   
/**  
 * @param array $weight 权重  例如array(&#39;a&#39;=>10,&#39;b&#39;=>20,&#39;c&#39;=>50)  
 * @return string key   键名   
 */  
function roll($weight = array()) {   
 $roll = rand ( 1, array_sum ( $weight ) );   
 $_tmpW = 0;   
 $rollnum = 0;   
  foreach ( $weight as $k => $v ) {   
  $min = $_tmpW;   
   $_tmpW += $v;   
   $max = $_tmpW;   
   if ($roll > $min && $roll <= $max) {   
    $rollnum = $k;   
    break;   
   }   
 }   
  return $rollnum;   
}   
 
$row=roll(array(&#39;a&#39;=>10,&#39;b&#39;=>20,&#39;c&#39;=>50));   
echo $row;   
?>