这事得从一道面试题说起：
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<?php
$a = "aabbzz";
$a++;
echo $a; //aabcaa
一开始觉得结果应该是 z 的 ASCII 下一位 {
后来在 php 手册上找到了答案，引用如下：
PHP follows Perl’s convention when dealing with arithmetic operations on character variables and not C’s. For example, in PHP and Perl $a = ‘Z’; $a++; turns $a into ‘AA’, while in C a = ‘Z’; a++; turns a into ’[’ (ASCII value of ‘Z’ is 90, ASCII value of ’[’ is 91). Note that character variables can be incremented but not decremented and even so only plain ASCII alphabets and digits (a-z, A-Z and 0-9) are supported. Incrementing/decrementing other character variables has no effect, the original string is unchanged.
php 在处理字符串的递增时，沿袭了 Perl 的风格，z 的下一位是 a