ack! why are you using a function to display all of this again? quit that ;) you only need the original IF statement you have, you don't need to create a function to echo anything.

functions are useful when you're going to be re-using code. they're unnecessary in your case. also, on why your variable wouldn't work -- you need to learn a little about variable scope. if you define a variable in the global scope (outside of a function), then it isn't available inside of the function. you could use the global keyword to make it available, but you'll have to read the link if you want to continue doing that.

anyway, this is what you should be trying to do -- I've removed your function once again and I've even cleaned up the print statement you have to make it readable.

PHP Code:
<?php include("dbconnection.php"); ?>
<div style="float: left; width: 100%; margin: 0px 0px 0px 0px; background-color: #7C7C7C; border: 1px solid #A5A498; border-width: 1px 1px 0px 1px;">
    <span style="float: left; padding: 2px 10px 0px 12px; color: #FFFFFF; font-family: arial; font-weight:bold; font-size: 13px;">Login</span>
</div>
<div style="float: left; width: 100%; height: 120px; background-color: #B4B3A9; border: 1px solid #A5A498; margin: 0px 0px 0px 0px; overflow-x: hidden; overflow-y: auto;">

<?php if(isset($_COOKIE['ID_my_site'])){ ?>
<span style="float: left; text-align: left; padding: 5px 5px 5px 5px;">
  Welcome <span id="myusername">{$myusername}</span>!<br />
  Visit your <a style="text-decoration: none;" href="login/member.php" onmouseover="this.style.textDecoration='underline';" onmouseout="this.style.textDecoration='none';" >member's stat</a><br />
  <a style="text-decoration: none;" href="login/logout.php" onmouseover="this.style.textDecoration='underline';" onmouseout="this.style.textDecoration='none';" >Logout</a>
</span>
<?php }else { ?>
<table border="0">
        <tr>
            <td>Administrator:</td><td><input type="checkbox" id="adminid" name="admin" style="float: left; border: 1px solid #7C7C7C; background-color: #7C7C7C;" value="no" onClick="if(this.value=='no'){this.value='yes'; document.getElementById('register').style.display = 'none';} else{this.value='no';document.getElementById('register').style.display = 'block';};" /></td>
       </tr>
<?php echo $writeemptyfield; ?>
<?php echo $writeusernoexist; ?>
        <tr>
            <td>Username:</td>
            <td><input type="text" name="username" id="username" style="float: left; border: 1px solid #7C7C7C; font: 9px verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif; width: 100px; height: 15px;" value="User Name" onBlur="if (this.value == '') this.value = 'User Name';" onFocus="if (this.value == 'User Name') this.value = '';" /></td>
       </tr>
<?php echo $writewrongpassword; ?>        
        <tr>
            <td>Password:</td><td><input type="password" style="float: left; border: 1px solid #7C7C7C; font: 9pt verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif; width: 100px; height: 15px;" name="pass" id="pass" /></td></tr>
        <tr>
            <td><a id="register" href="login/register.php" style="float: left; display: block; background-color: #7C7C7C; text-align: left; padding: 0px 0px 0px 5px; width: 80px; height: 20px; text-decoration: none; color: #ffffff; border: 1px solid #7C7C7C; font-weight: bold;" onMouseOver="this.style.backgroundColor='#B4B3A9'" onMouseOut="this.style.backgroundColor='#7C7C7C'">Register</a></td><td align="right"><input type="submit" style="float: left; border: 1px solid #7C7C7C; background-color: #7C7C7C; color: #ffffff; font: 9pt verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif; font-weight: bold; width: 100px; height: 20px;" name="submit" onMouseOver="this.style.backgroundColor='#B4B3A9'" onMouseOut="this.style.backgroundColor='#7C7C7C'" value="Login" /></td>
        </tr>
    </table>
<?php } ?>
</div>
logically, this will do exactly what you've done before. it's just better structured and written in a way that should be more maintainable than what you have done.

you may also want to look into using a stylesheet instead of having all of that inline CSS. as far as maintainability goes, your HTML will be incredibly difficult to change in the future if you choose to do so.