php – 如何将数字相乘,直到达到一位数字并计算数字？

它的描述如下：

persistence(39) == 3 // because 3*9 = 27,2*7 = 14,1*4=4
// and 4 has only one digit

persistence(999) == 4 // because 9*9*9 = 729,7*2*9 = 126,// 1*2*6 = 12,and finally 1*2 = 2

persistence(4) == 0 // because 4 is already a one-digit number

我只能这样做：

$array = str_split(39);

    foreach ($array as $key => $value) {
        echo $array[$key]*$array[$key+1];
    }

接下来我很困惑

解决我的问题的任何解决方案？

$array = str_split('999'); //Your string
$j=0; //Counter for counting the number of iteration
while (count($array)>1){ //When more than 2 indexes in array
 for($i=0;$i<count($array);$i++){ //Iterate through all permutations
     $array = array_product($array); //Multiplies all numbers in array
     $array = str_split($array); //Split the array up again
     $j++; //Increment counter(as literal as I can sound)
 }
}
echo $j; //Print out the number of times