Description:
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After filling variable $_SESSION["user_id"] I was unable to fill any other new $_session variable.

PHP version 4.3.1

Reproduce code:
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session_start(); 
if (!empty($_REQUEST["auth_user"])) $_SESSION["user"]=$_REQUEST["auth_user"]; 
if (!empty($_REQUEST["auth_pass"])) $_SESSION["pass"]=$_REQUEST["auth_pass"]; 
if (!empty($_SESSION["userRow"])) return;

$result=execQuery("SELECT * FROM table_name WHERE user_name=\"".$_SESSION["user"]."\" AND user_password=\"".md5($_SESSION["pass"])."\" AND user_active=1"); 

$_SESSION["userRow"]=mysql_fetch_assoc($result);


$_SESSION["user_id"] = $_SESSION["userRow"]["user_id"];
$_SESSION["user_num"] = $_SESSION["userRow"]["user_num"];


Expected result:
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$_SESSION["user_num"] should be filled with the content of $_SESSION["userRow"]["user_num"]

Actual result:
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$_SESSION["user_num"] is not filled.