我覺得這樣寫會(huì)清楚一些：

<?php
$str1 = "01 ";
$str1++;
var_export($str1);//得到'01 '

$str2 = "01";
$str2++;
var_export($str2);//得到2

參見這里的解釋：

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.

大意是說，使用自增/自減操作字符串時(shí)，可以遞增但不能遞減，另外只支持(字符串的末位為)純 ASCII 字母和數(shù)字 (a-z、a-z 和 0-9)。（另外注意這段中文文檔和英文的對(duì)不上，暫且以英文為準(zhǔn)）