php://memory
php://memory 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://memory always stays in memory, so it may reach the memory limit of PHP. On the other hand, php://tmp does the same, but overflows to disk when needed.
<?php
file_put_contents('php://memory', $data);
$data = file_get_contents('php://memory');
?>