Empty string problem

Following is a snippet of code that I am trying to debug. Most of the time, the variable $sn (set on the last line), correctly contains the variable $this_customer_num. On some occasions, however, it does not contain $this_customer_num. I cannot figure out what may be happening that this value is missing around 1-10% of the time. <?php $memberchecker=mysql_query("SELECT id, password, customer_num FROM logins WHERE email = '$email'" ); //email is unique and not null if (!$memberchecker) { //handle error } else { if (mysql_num_rows($memberchecker) >= 1) {

       while ($row = mysql_fetch_array($memberchecker)) {

           $this_login_password=$row[password];

           if ($this_login_password==$password) {
           //$password is a form variable (yes register_globals is on...*sigh*)

               $this_customer_num=$row[customer_num];

               $companycheck=mysql_query("SELECT priority, cash_price FROM customers WHERE customer_num='$this_customer_num'"); //customer_num is primary key, so unique and not null

               if (!$companycheck) {

                   //handle error

               } else {

                   if (mysql_num_rows($companycheck) == 1) {

                       while ($cow = mysql_fetch_array($companycheck)) {

                           $id=$row[id];
                           $priority=$cow[priority];
                           $cp=$cow[cash_price];
                           $sn=date(Sz).$this_customer_num.time();

?> Before anyone yells at me, no I did not write this code. I first thought there might be a problem with the array keys not being quoted, but if that were the case, and an empty string were assigned to $this_customer_num, then I should not be able to pass the second query, since there are no empty string customer_num entries (all of them are at least 9 characters in length). I have been unable to reproduce an instance where $sn is missing the $this_customer_num variable, and I'm stuck in a kind of political situation where I can only make recommendations, and not change the code myself. Suggestions on where to start looking? Thanks in advance.