Description:
------------
When using readfile or fread in combination with 
session_start php seems to allocate memory for the whole 
filesize. Without session_start it doesn't. 
 
Replacing readfile by a fread loop doesn't make any 
difference. 
 

Reproduce code:
---------------
<?php
 //error appears:
 session_start();
 readfile('big.zip'); //big.zip > memory_limit
?>

<?php
 //working fine:
 readfile('big.zip'); //big.zip > memory_limit
?>

<?php
  //fread version, same error
  session_start();
  $fp = fopen('big.zip');
  while($d = fread($fp,4096)) {
    echo $d;
    flush();
  }
?>

Expected result:
----------------
Memory consumation should stay much bellow the whole 
filesize as it does without sessions. 

Actual result:
--------------
apache error.log: 
Allowed memory size of 16777216 bytes exhausted (tried to 
allocate 138188800 bytes)