读一下文档吧：doc

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 characters (a-z and A-Z) are supported. Incrementing/decrementing other character variables has no effect, the original string is unchanged.

可以用这种方式：

for($i=97;$i<122;$i++){

echo chr($i).”n”;

}

或者：

for($i=ord(‘a’);$i

echo chr($i).”n”;

}