Session and header() errrors

Why do the errors occur while running the scripts below? It seems that the errors occured at the part of session and header(). Please give me some advice! Thank you! Mike System: PHP4.06 + Apache 1.3.20 Win32 + Win98 (the session configurations are default in php.ini) when the 1.php is running, the IE shows errors info as below : ----------------- Warning: Cannot send session cookie - headers already sent by (output started at c:\program files\apache group\apache\htdocs\web1\html-head.inc:9) in c:\program files\apache group\apache\htdocs\web1\1.php on line 3 Warning: Cannot send session cache limiter - headers already sent (output started at c:\program files\apache group\apache\htdocs\web1\html-head.inc:9) in c:\program files\apache group\apache\htdocs\web1\1.php on line 3 Warning: open(/tmp\sess_96ae897bcb501486860552d2df862863, O_RDWR) failed: m (2) in c:\program files\apache group\apache\htdocs\web1\1.php on line 3 session_id: 96ae897bcb501486860552d2df862863 Warning: Cannot add header information - headers already sent by (output started at c:\program files\apache group\apache\htdocs\web1\html-head.inc:9) in c:\program files\apache group\apache\htdocs\web1\1.php on line 9 ------------- The scripts of 1.php: (1.php and 2.php are under the same base directory) <? require("html-head.inc"); session_start(); $name="user"; session_register("name"); echo "session_id: ".session_id()."<br>"; sleep(10); header("Location: 2.php"); require("html-foot.inc"); ?> ----- The scripts of html-head.inc: <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>page title</title> </head> <body> ----- The scripts of html-foot.inc: </body> </html>