/*
 * 冒泡排序
 */
$arr = array(4,5,2,15,9);
$len = count($arr);

//从后往前冒泡  $i从0开始
for ($i = 0; $i < $len; $i++){
    for($j = $len - 1; $j > $i; $j--){
        if($arr[$j] < $arr[$j-1]){
            $tmp = $arr[$j];
            $arr[$j] = $arr[$j-1];
            $arr[$j-1] = $tmp;
        }
    }
}
//从前往后冒泡 $i从1开始
for ($i = 1; $i < $len; $i++){
    for ($j = 0; $j < $len - $i; $j++){
        if($arr[$j] > $arr[$j+1]){
            $tmp = $arr[$j+1];
            $arr[$j+1] = $arr[$j];
            $arr[$j] = $tmp;
        }
    }
}
print_r($arr);//Array ( [0] => 2 [1] => 4 [2] => 5 [3] => 9 [4] => 15 )