Warnings and captcha image

Hello,

From my current host's cPanel, I did back-ups of my web site and database and transferred everything to my second new host. My web site which is still hosted by my current host works perfectly fine. There's no errors and the captcha image works. As you can see, my current host is very good but too expensive for me. A few weeks ago, I moved to another webhost and warnings appeared and they couldn't help me. And now with my second new host I also have the exact same warnings and I'm getting a bit tired to figure them out. I looked for spaces before and after <?php and ?> and couldn't find any. I googled to look for help and came across a post where people suggest placing ob_start(); but it didn't work either.

I don't understand why I keep getting those warnings since I didn't change anything at all... (except for the config file of course!)

Warnings
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/shang/public_html/header.php:1) in /home/shang/public_html/header.php  on line 54

Warning: Cannot modify header information - headers already sent by (output started at /home/shang/public_html/header.php:1) in /home/shang/public_html/header.php on line 63

Warning: Cannot modify header information - headers already sent by (output started at /home/shang/public_html/header.php:1) in /home/shang/public_html/header.php on line 64

Warning: Cannot modify header information - headers already sent by (output started at /home/shang/public_html/header.php:1) in /home/shang/public_html/header.php on line 65
  
My header.php

Line 54: if(!isset($_SESSION)) session_start();
Line 63: Header('Cache-Control: private, no-cache, must-revalidate, max_age=0, post-check=0, pre-check=0');
Line 64: header ("Pragma: no-cache"); 
Line 65: header ("Expires: 0"); 

Captcha image
It doesn't work at all.

I already asked help from my new host but they don't seem to be able to help me... They changed my permissions and said GD support is enabled. I even told them that "Warning: session_start()" is a server issue configuration.

I was also wondering if different versions of PHP and MySQL could be the cause of those warnings...

Current host
PHP version 5.2.9
MySQL version 5.0.81-community-log

New  host
PHP version  5.2.12
MySQL version  5.1.44-log

I find that very odd and wonder if anyone could help me.

Thank you,