<?php

for ($length=1; $length<=5;$length++) {

$half1=round(($length+1)/2);  //ошибка тут!
$half2=round($length/2);      //верно
$half3=round(($length+1)>>1); //верно
$half4=$length+1>>1;          //верно

echo "length:".$length." => ".$half1." ".$half2." ".$half3." ".$half4.PHP_EOL;
}