Performance: include() vs include_once()

--------------------------------------
MAIN FILE (test_include.php)
--------------------------------------

<?php
$loop_time = 3;
include('include_file1.php');
include('include_file2.php');
include('include_file3.php');

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file1.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(large_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file1.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(large_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;

echo PHP_EOL;

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file2.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(small_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file2.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(small_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;

echo PHP_EOL;

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file3.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(empty_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file3.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(empty_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


/* REVERSE */
echo PHP_EOL,'reversing order and retesting ...',PHP_EOL,PHP_EOL;
/***********/

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file3.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(empty_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file3.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(empty_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;

echo PHP_EOL;

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file2.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(small_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file2.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(small_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;

echo PHP_EOL;

$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include_once('include_file1.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include_once(large_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;


$iterations = 0;
$start_time = microtime(true);
$duration = 0;
while ($duration <= $loop_time) {
    include('include_file1.php');
    $iterations++;
    $duration = microtime(true) - $start_time;
}
echo 'include(large_file): ',$iterations,' iterations in ',$duration,' seconds',PHP_EOL;

?>

---------------------------------
LARGE INCLUDE (include_file1.php)
---------------------------------
<?php
if (defined('INCLUDE_FILE2')) return;

define('INCLUDE_FILE2',true);
sleep ($loop_time);

/* COPY / PASTE / REPEAT the following line about 1250 times to increase file size */
/* Use up a bunch of memory or disk space to make the include more "costly" */
/* Use up a bunch of memory or disk space to make the include more "costly" */
/* ... /*
?>

---------------------------------
SMALL INCLUDE (include_file2.php)
---------------------------------
<?php
if (defined('INCLUDE_FILE2')) return;

define('INCLUDE_FILE2',true);
sleep ($loop_time);
?>

----------------------------------
EMPTY INCLUDE (include_file3.php)
----------------------------------


----------------------------------
TEST RESULTS
----------------------------------
D:\www\manual>php test_include.php
include(large_file): 12118 iterations in 3.015625 seconds
include_once(large_file): 196366 iterations in 3.015625 seconds

include(small_file): 34986 iterations in 3.015625 seconds
include_once(small_file): 222284 iterations in 3.015625 seconds

include(empty_file): 68155 iterations in 3.015625 seconds
include_once(empty_file): 220433 iterations in 3.015625 seconds

reversing order and retesting ...

include_once(empty_file): 192640 iterations in 3.015625 seconds
include(empty_file): 68381 iterations in 3.015625 seconds

include_once(small_file): 222204 iterations in 3.015625 seconds
include(small_file): 34714 iterations in 3.015625 seconds

include_once(large_file): 226388 iterations in 3.015625 seconds
include(large_file): 9577 iterations in 3.015625 seconds

D:\www\manual>