<?php
$words = [
'aaaaaaa',
'bb',
'ccc',
'ddddd',
'e',
];
usort($words, function($a, $b) { return strlen($a) <= strlen($b); });
var_dump($words);
実行結果
array(5) {
[0] =>
string(7) "aaaaaaa"
[1] =>
string(5) "ddddd"
[2] =>
string(3) "ccc"
[3] =>
string(2) "bb"
[4] =>
string(1) "e"
}