Conversion between arrays and strings in php

Convert array to string:

implode('!', $arr);//Convert 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 with !, parameter Number one cannot be ""

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