php://tmp
php://tmp is a special read-write stream that gives access a space in memory. It is removed at the end of execution of the current code, so this stream is ideal for temporary and fast storage.
php://tmp stays in memory, and, when reaching a certain level, it overflows to disk. This allows this space to go beyond the maximum memory limit of PHP, without stopping the script.
<?php
file_put_contents('php://tmp', $data);
$data = file_get_contents('php://tmp');
?>