Array to string:

implode('!', $arr);//Convert a one-dimensional array to ! Separate and combine into a string, parameter one can be ""

Convert string to array:

explode('!', $str);//Split the string into a one-dimensional array using !, parameter one cannot be ""

str_split($str, 3);//Split the string into an array. The second parameter splits the string every 3 characters from left to right. The last one is not enough. Count how many there are.