如何根据php中的具体数字插入特定的字符串？

Question

i have a situation that need to insert specific string according to specific number. for example the number is 4 and i need 4 of specific string (in this case is '?' ) like this: number 5 so $str = '?????'. in another word i have a array like this ['first item','second item', 'third item'] . i need to count items in this array, that can accomplished by count() and according to that number insert ? . so the final string should be '?,?,?' with ',' as separator.