memory.php

<?php
$with_unsused = false;

define('LIMIT', 1000000);

class MyClass {
    public function test() {}
}

function callSomething() {
    (new MyClass)->test();
}

function callSomethingWithVariable() {
    $var = (new MyClass)->test();
}

// Alocate memory for integer, so it does not has effect on $diff
$mem_before = 0;
$mem_after = 0;
$i = 0;

$i = 0;
$mem_before = memory_get_usage($with_unsused);

for (; $i < LIMIT; $i++) {
    callSomething();
}

$mem_after = memory_get_usage($with_unsused);

echo "(new MyClass)->test();", PHP_EOL;
echo "Before: ", $mem_before, PHP_EOL;
echo "After:  ", $mem_after, PHP_EOL;
echo "Diff:   ", ($mem_after - $mem_before), PHP_EOL;
echo PHP_EOL;

$i = 0;
$mem_before = memory_get_usage($with_unsused);

for (; $i < LIMIT; $i++) {
    callSomethingWithVariable();
}

$mem_after = memory_get_usage($with_unsused);

echo '$var = (new MyClass)->test();', PHP_EOL;
echo "Before: ", $mem_before, PHP_EOL;
echo "After:  ", $mem_after, PHP_EOL;
echo "Diff:   ", ($mem_after - $mem_before), PHP_EOL;

/*
 ❯ php --version
PHP 7.1.8 (cli) (built: Aug 29 2017 14:16:17) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies

 ❯ php memory.php
(new MyClass)->test();
Before: 377480
After:  377480
Diff:   0

$var = (new MyClass)->test();
Before: 377512
After:  377512
Diff:   0

with:
PHP 5.6.30-0+deb8u1 (cli) (built: Feb  8 2017 08:50:21)

(new MyClass)->test();
Before: 232600
After:  232640
Diff:   40

$var = (new MyClass)->test();
Before: 232688
After:  232728
Diff:   40
*/

updated with php5.6