$ array = str_split （ '999'）; //你的字符串
 $ j = 0; （count（$ array）> 1）{//当数组
中有多于2个索引时（$ i = 0; $ i  $ array = array_product（$ array）; //将数组中的所有数字相乘
 $ array = str_split（$ array）; //再次拆分数组
 $ j ++; //增加计数器（按照我听到的文字）
} 
} 
 echo $ j; //打印次数
 

its description like this:

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

I can only make like this :

$array = str_split(39);

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

The next I'm confused

Any solution to solve my problem?

$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