MySQL / php help needed

[Vitesse]

Would someone be able to just check my code here and see where i've gone wrong.

PHP Code:

$query=" SELECT * FROM quartertime WHERE user='$user'";   //Query the DB to check if user exists
$result=mysql_query($query);
$num=mysql_numrows($result);
if ($result !=0){                                    //if record already exists
             //amend it
} else {

// Create new record

}
//Rest of script 


Basically i just want to see if one or more records exist that match a username held in $user

the if/else is so if the record exists it amends all records with 'user'=$user
otherwise it makes a new record with their details. The code for adding/amending is already done and working just cant get the If/Else right

Many thanks