Sorry to ask a silly question, but are we sure that the person who saw the error is on the same Moodle site as you? Can they reproduce the error? Can you reproduce the error?
If the answers to these questions are YES-YES-YES then we can proceed as follows to try to force Moodle and PHP to play nicely together.
This error message comes from "lib/setuplib.php", in the setup_validate_php_configuration() function (around line 638)
if (ini_get_bool('session.auto_start')) {
print_error('sessionautostartwarning', 'admin');
}
So although you and I believe that PHP info on your your Moodle site is telling us that session.auto_start is "off", it has somehow been switched on by the time this bit of PHP runs.
You could try adding the following line of PHP code to your Moodle config.php file:
@ini_set('session.auto_start', '0');
Put it right very near the top of the file, just after the "<?php" line
If that doesn't work, you could try creating a file called ".htaccess" (Moodle Docs: htaccess) in the Moodle scripts folder on your server (i.e. the same folder that contains your Moodle config.php file), and put the following line in the ".htaccess" file:
php_flag session.auto_start 0
If that doesn't work, create a file called "php.ini" in your Moodle folder and put the following line in it:
session.auto_start = 0
If that doesn't work, please contact your server administrator, or your web host help desk, and show them the message that Moodle showed you, and insist that Moodle thinks session.auto_start is on, but you want to force it off.
if any of the above methods works, please let us know 
regards
Gordon
