Why doesn't this code simply print out the letters A to Z?

Question

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

This code snippet provides the following output (newlines replaced with spaces):

a b c def g h i j k l m no 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 dze ea eb ec ed ee ef eg eh ei ej ek el em en eo ep eq er es et eu ev ew ex...to yz

Answer 1

Because once 'z' is reached (which is a valid result in the range, $i increments it to the next value in the sequence), the next value will be 'aa'; alphabetically, 'aa' is < 'z'，因此永远不会满足比较

for ($i = 'a'; $i != 'aa'; $i++) 
    echo "$i\n";

Answer 2

From Documentation:

From comments: -
Also note that <= is a lexicographic comparison, so 'z' 1 ≤ 'z'. (Because 'z' 1 = 'aa' ≤ 'z'. But 'za' ≤ 'z' is false for the first comparison.) For example, $ i == 'z' will do.

Example.