"marginally" definitely means "marginally" in this case.
We're talking thousands of variable replacements (double quoted) before you'll notice ANY performace decrease, but you are correct, you do get some miniscule amount of gain from breaking out of quotes to substitute.

PHP Code:
<?php

function microtimeAsDouble()
{
    
$nowTime explode(" "microtime());
    return (
$nowTime[0] + $nowTime[1]);
}

// turn on automatic flushing:
ob_implicit_flush();

// seed random number generator:
srand((double)microtime()*10000);

// set memory limit to 64 megs.
ini_set('memory_limit''64M');

// how many?
$iterations 10000;

$b "test";

$startTime microtimeAsDouble();
for (
$i=0$i<=$iterations$i++) {
    
$a '---'$b .'---';
}
$singleTime microtimeAsDouble() - $startTime;

echo 
"Single: "$singleTime ." seconds.<br /><hr />";


$startTime microtimeAsDouble();
for (
$i=0$i<=$iterations$i++) {
    
$a "---$b---";
}
$doubleTime microtimeAsDouble() - $startTime;

echo 
"Double: "$doubleTime ." seconds.<br /><hr />";

?>
for 100,000 iterations, I'm getting (~):
Code:
Single: 0.38587403297424 seconds.
Double: 0.50207698345184 seconds.
S