<?php
$test = 'OLOLO';

$start = microtime(true);
$x="test".$test;
echo microtime(true)-$start."\n";

$start = microtime(true);
$x="test $test"; 
echo microtime(true)-$start."\n";

$start = microtime(true);
$x="test";$x.=$test;
echo microtime(true)-$start."\n";