25. 实现strStr()

<?php
class Solution {
    public function strStr($haystack, $needle) {
        for ($i = 0; $i <= strlen($haystack) - strlen($needle); $i++) {
            $str = substr($haystack, $i, strlen($needle));
            if ($str == $needle) {
                return $i;
            }
        }
        return -1;
    }
}