example.php

<?php

function microtime_float()
{
    list($usec, $sec) = explode(" ", microtime());
    return ((float)$usec + (float)$sec);
}

$time_start = microtime_float();

$string = "testing string 123";
$number = 2000;
$text = "fdddddd dewwwwww";

for($i = 0; $i <= 100000; $i++) {
        $string .= " adding some text " . $number . $text;
}

$time_end = microtime_float();
$time = $time_end - $time_start;
echo "echo double quotes took $time\n";


$time_start = microtime_float();

$string = "testing string 123";
$number = 2000;
$text = 'fdddddd dewwwwww';

for($i = 0; $i <= 100000; $i++) {
        $string .= ' adding some text ' . $number . $text;
}

$time_end = microtime_float();
$time = $time_end - $time_start;
echo "echo single quotes took $time\n";

Jeremys-MacBook-Pro:opg-core-docker jeremyquinton$ php example.php 
echo double quotes took 0.018709182739258
echo single quotes took 0.023837804794312

Jeremys-MacBook-Pro:opg-core-docker jeremyquinton$ php example.php 
echo double quotes took 0.019231081008911
echo single quotes took 0.019197940826416

Jeremys-MacBook-Pro:opg-core-docker jeremyquinton$ php example.php 
echo double quotes took 0.019853115081787
echo single quotes took 0.019999980926514