micro optimization!! each test now involves many tests, so average is calculated and indicated in comments. one test was run at a time.

optimization_test.php

<?php
$foo = 'foo';
$bar = 'bar';
$foobar = 'foobar';

$string = '';
$tests = array();

for ($z = 0; $z<1000; $z++){
	$startTime = microtime(true);
	for ($i = 0; $i<1000; $i++) {
		// $string .= 'this ' . ' is ' . $foo . ', this is ' . $bar . ' and this is ' . $foobar; // 0.4931318783 Milliseconds
		// $string .= "this is $foo, this is $bar and this is $foobar";  // 0.4040865907 Milliseconds
		// $string .= "this is {$foo}, this is {$bar} and this is {$foobar}"; // 0.4096872842 Milliseconds
	}
	$tests[] = number_format(( microtime(true) - $startTime), 10) * 1000;
}

echo array_sum($tests) / count($tests);