php 冒泡排序

$a=array('3','8','1','4','11','7');
 print_r($a);
 echo '<br/>';
 $len=count($a);
 for($i=1;$i<$len;$i++){
    for($j=$len-1;$j>=$i;$j--){
        if($a[$j]>$a[$j-1]){
             $x=$a[$j];
             $a[$j]=$a[$j-1];
             $a[$j-1]=$x;
print_r($a);
        }
    }
}