display pre selected value in dropdown list

Question

I have 2 tables Category and Product

category

id     name 
1   categoryone 
2   categorytwo

product

id  categoryid  product
1     1            P1

I have a dropdownlist that displays the category , but if any category is present in product table then it should get displayed as a pre selected value from dropdown list, and other categories should get displayed normally

<?
$category="SELECT * FROM `category` ";
$resultcategory = mysqli_query($con, $category);
if(mysqli_num_rows($resultcategory)>0)
    {
        while($rowcategory = mysqli_fetch_assoc($resultcategory))
            {?>

                <option value="<? echo $rowcategory['id']; ?>"><? echo $rowcategory['name']; ?></option>
            <?}
    }
?><?

$category="SELECT * FROM `category` ";
$resultcategory = mysqli_query($con, $category);
if(mysqli_num_rows($resultcategory)>0)
    {
        while($rowcategory = mysqli_fetch_assoc($resultcategory))
            {?>

                <option value="<? echo $rowcategory['id']; ?>"><? echo $rowcategory['name']; ?></option>
            <?}
    }
?>   

Can anyone tell how it can be done

Answer 1

First you will need to rebuild your select string to something like this:

$category = "SELECT category.*, product.categoryid".
 "FROM category, product".
    "WHERE category.id= product.categoryid";

And then add a check in PHP code to see if they match like this

 while($rowcategory = mysqli_fetch_assoc($resultcategory))
            {
                <option value="<?php echo $rowcategory['id']?>" <?php if ($rowcategory['id'] == $rowcategory['categoryid']): ?> selected="selected" <?php endif; ?>></option>
            }

There is probably a million of syntax errors in my code, but idea should be clear.