is each session call is separate call to server - php

Question

Keeping in mind of server load, I want to know whether each call to session is a separate call to server which increases server load?
Let me explain it little bit. Suppose in each php page, I have to set 10 session data and have to call 10 session data also like this:
$_SESSION['a']=1;
$_SESSION['b']=2;
$_SESSION['c']=3;
$_SESSION['d']=4;
$_SESSION['e']=5;
$_SESSION['f']=6;
$_SESSION['g']=7;
$_SESSION['h']=8;
$_SESSION['i']=9;
$_SESSION['j']=10;
echo $_SESSION['a'];
echo $_SESSION['b'];
echo $_SESSION['c'];
echo $_SESSION['d'];
echo $_SESSION['e'];
echo $_SESSION['f'];
echo $_SESSION['g'];
echo $_SESSION['h'];
echo $_SESSION['i'];
echo $_SESSION['j'];
setting of above 10 sessions and outputting the same, will these be 20 separate calls to server or by session_start(), whole session data is loaded at once during page loading?
Also want to know how to store multidimensional associative array in session? Suppose I want to set the above 10 session data like this:
$_SESSION['mydata']=array(
a=>1,
b=>2,
c=>3,
d=>4,
e=>5,
f=>6,
g=>7,
h=>8,
i=>9,
j=>10
)
echo $_SESSION['mydata']['a'];
By the above example I can reduce server loads if each session call is separate call to server.
Anyone clear me what's going on when we set and get session data?

Answer 1

Here is the breakdown of a session
session_start(); either creates a session file on the hard drive or accesses one if it is already there. It is a serialized array
This array gets loaded into memory and is manipulated within memory until the script finishes executing or you can force it to re-write the file with session_write_close()
You can add/remove as much data as you wish after you have started the session because it is no different than manipulating an array.
The heavy lifting is done at session_start and session_write_close because that is when PHP actually has to access the hard drive
In theory each user can have a 4GB session file on a FAT32 server, or larger on NTFS server, but I am sure you can imagine the horrendous performance issues that would arise.

Answer 2

The whole execution of your script is done in one request. By setting $_SESSION['foo']='bar' and $_SESSION['baz']='biz' you don't set two different sessions but rather two session variables.
So the whole session is loaded on script start, not page load (that's a client-side term).
It's all being done with cookies.
EDIT, to answer your comment:
As others mentioned (see Mark Baker's comment) the session is loaded once, right before the script kicks off, and stored once after script end (unless you explicitly call session_write_close())
Sure, size matters. Apparently, the bigger the session the longer it will take to be loaded/stored. But that would be the last thing you would bother to optimize.

Answer 3

Benchmark for session
For $_SESSION['mydata']['key'] = 'value' :
session_start();
$time_begin = microtime(true);
$mem_start = memory_get_usage();
for ($i=0; $i < 100000; $i++) {
$_SESSION['mydata']["a$i"] = $i;
}
foreach ($_SESSION['mydata'] as $key => $value) {
$r = $value;
}
$mem_end = memory_get_usage();
$time_end = microtime(true);
echo "Time :";
echo $time_end - $time_begin;
echo "<br>";
echo "Memory :";
echo $mem_end - $mem_start;
My result about $_SESSION['mydata']['key']:
Time :0.57849311828613
Memory :464
For $_SESSION['key'] = 'value' :
<?php
session_start();
$time_begin = microtime(true);
$mem_start = memory_get_usage();
for ($i=0; $i < 100000; $i++) {
$_SESSION["a$i"] = $i;
}
for ($i=0; $i < 100000; $i++) {
$r = $_SESSION["a$i"];
}
$mem_end = memory_get_usage();
$time_end = microtime(true);
echo "Time :";
echo $time_end - $time_begin;
echo "<br>";
echo "Memory :";
echo $mem_end - $mem_start;
My result about $_SESSION['mydata']:
Time :0.47217416763306
Memory :176
EDITED : In the first PHP Script memory spented by the mydata array and the other mydata's keys.So, second way is greater again.
So , using $_SESSION['key'] = 'value' is greater than $_SESSION['my']['key'] = 'value'.
This can be a reason that when you want to set a value to $_SESSION['my']['key'], php should get $_SESSION['my'] and after that it can be create new var. in the $_SESSION['my']['key']