PHP - Erroneous Alphabet Loop

问题:

Can Anyone Explain me why :

    <?php
    for ($i = 'a'; $i <= 'z'; $i++){
    echo "$i ";
    }
    ?>

Why its Output is :

a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs bt bu bv bw bx by bz ca cb cc cd ce cf cg ch ci cj ck cl cm cn co cp cq cr cs ct cu cv cw cx cy cz da db dc dd de df dg dh di dj dk dl dm dn do dp dq dr ds dt du dv dw dx dy dz ea eb ec ed ee ef eg eh ei ej ek el em en eo ep eq er es et eu ev ew ex... on to yz

But its Working Fine With

    <?php
    for ($i = 'a'; $i != 'aa'; $i++){
    echo "$i ";
    }
    ?>

The loop seems to run over and over again until it reaches "zz".

If I give $i<"aa" it should work fine but instead it outputs only "a".

回答1:

See the manual for the increment operator:

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.

回答2:

You can't do a less than ($i < "aa") operator on a string.

It will get converted to a numerical type and the results are usually unpredictable.