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
$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
