HELP!!! Defining variables with form values

[RossBryan]

How would i go about correctly defining the following variables that i am taking in from a form.

<?php require ("design/top.php"); ?>

<title>Care2Share - Register</title>
    
<div id= 'left'>
    
<?php
$form = "<form action='register.php' method='post'>
<table cellspacing='10px'>
<tr>
    <td><input type='text' id='usernameboxReg' name='user' tabindex='1' value='Username' class='textbox' onfocus='usernameboxReg_focus();' onblur='usernameboxReg_blur();'></td>
    <td><font size='-1' color='#FF0000'>(*required)</font></td>
</tr> 
<tr>
    <td><input type='text' id='emailbox' name='email' tabindex='2' value='Email' class='textbox' onfocus='emailbox_focus();' onblur='emailbox_blur();' /></td>
    <td><font size='-1' color='#FF0000'>(*required)</font></td>
</tr>
<tr>
    <td><input type='text' id='passwordboxReg' name='pass' tabindex='3' value='Password' class='textbox' onfocus='passwordboxReg_focus();' onblur='passwordboxReg_blur();' /></td>
    <td><font size='-1' color='#FF0000'>(*required)</font></td>
</tr>
<tr>
    <td><input type='submit' name='registerbutton' class='button' value='Register'/></td>
</tr>

</table>
</form>";

if (isset($_POST['registerbutton']))
{
    $user = $_POST['user'];    
    $email = $_POST['email'];
    $password = $_POST['pass'];
    
    if ($user == "Username")
        $user = "";
    if ($email == "Email")
            $email = ""; 
    
        if ($user && $email && $password)//ensures these three values are submitted 
        {
            if (strstr ($email, "@")&& strstr($email, "."))
            {
                require ("design/connect.php");
                
                $query = mysql_query("SELECT * FROM users WHERE user='$user'");//select everything from the users table where everything =the username provided
                $numrows = mysql_num_rows($query);//does the query say the username is taken?
                
                
            }
        
            else
                echo "That is not a valid email.$form";
                
        }
        else 
            echo "You did not fill in the required fields.$form";     

        }

else
    echo "$form";

?>
    
</div>
    
 <?php require ("design/bottom.php"); ?>

Ive tried to apply the (isset ()) function for example: (isset($_POST=['user']))
but it doesnt seem to be defining my new variable. could anyone possible lend me some advise.

[webdev1958]

if(isset($_POST['user'])) {

   // do this

}