<pre>
<?php

$num = 10000;
$chunk = str_repeat('1234567890', 1);

$start_time = microtime(true);
$start_mem = memory_get_usage();

$ret = '';
$arr = array();
for ($i=0; $i<$num; $i++) {
    $arr[] = $chunk;
    $ret.=$chunk;
}

printf("\nwarmup job: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);

$ret = '';
$arr = array();

printf("\nwarmup flush: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);


$start_time = microtime(true);
$start_mem = memory_get_usage();

$ret = '';
$arr = array();
for ($i=0; $i<$num; $i++) {
    $arr[] = $chunk;
}

$ret = join ('', $arr);

printf("\narray job: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);

$ret = '';
$arr = array();

printf("\nflush: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);

$start_time = microtime(true);
$start_mem = memory_get_usage();
for ($i=0; $i<$num; $i++) {
    $ret.= $chunk;
}

printf("\nconcat job: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);

$ret = '';
$arr = array();

printf("\nflush: time: %f | memory: %d", microtime(true)-$start_time, memory_get_usage() - $start_mem);

?>
</pre>