PHP mySql error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE ProductId='x'' at line x

I think it is OK to quote integer fields, but not necessary.  However an extraneous comma anywhere in the query string is trouble.

A good strategy for writing queries is to create a separate variable for the query string.  Then you can visualize the query string in case there is an error.  Here is a code example of how I might do it.

// NOT THIS...
 mysql_query("UPDATE products SET ProductName='$ProductName', SupplierId='$SupplierId' ,CategoryId='$CategoryId', QuantityPerUnit='$QuantityPerUnit',UnitPrice='$UnitPrice',UnitsInStock='$UnitsInStock',UnitsOnOrder='$UnitsOnOrder',ReorderLevel='$ReorderLevel',Discontinued='$Discontinued'
,LastModifiedDateTime='$LastModifiedDateTime'  WHERE ProductId='$ProductId'") 

// BUT THIS
$sql
=
"UPDATE products 
SET 
  ProductName          = '$ProductName'
, SupplierId           = '$SupplierId' 
, CategoryId           = '$CategoryId'
, QuantityPerUnit      = '$QuantityPerUnit'
, UnitPrice            = '$UnitPrice'
, UnitsInStock         = '$UnitsInStock'
, UnitsOnOrder         = '$UnitsOnOrder'
, ReorderLevel         = '$ReorderLevel'
, Discontinued         = '$Discontinued'
, LastModifiedDateTime = '$LastModifiedDateTime'  
WHERE 
  ProductId = '$ProductId'
"
;
$res = mysql_query($sql);
if (!$res)
{
    echo "FAIL: $sql ";
    echo "BECAUSE: " . mysql_error();
    /* HANDLE ERROR CONDITION HERE */
}

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Obviously this is untested code (I don't have your data base) but hopefully it makes for a good example of how to write very readable query strings.  Want to find your queries in a large script file?  Look for the equal sign in column 1.  Need to add columns to the query -- easy!  You might want to try deliberately introducing an error into the query and look at the error message output.  I think you'll find that "neatness counts" and pays some big benefits when you're debugging.

Best to all for a good New Year, ~Ray