<?php

    function cmp ($a, $b) {
         return strcmp(substr($a, 0, $num), substr($a, 0, $num));
    }

    $terms = ['123a', '123z', '123b',];
 	$num = 3;$thing=3;
    usort($terms, [$num, $thing, cmp]);
 
    print_r($terms);