<?php

$n = 0;
$m = 0;
echo '++ before:';
echo $n+ ++$n;
echo PHP_EOL;
echo '++ after:';
echo $m+ $m++;
echo PHP_EOL;
echo 'n:'.$n;
echo PHP_EOL;
echo 'm:'.$m;

++ before:2
++ after:1
n:1
m:1