search mysql using checkboxs?

I have been working on this for hours to no avail.

I'm trying to create an advanced search feature for my site using PHP and mysql.

the advanced search only has checkboxes in it.

the checkboxes look like this:

<form action="search.php"  method="POST" id="myForm"/>

<table width="100%" border="0" cellspacing="0" cellpadding="0">
  <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="small"/></td>
    <td>Small</td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="large"/></td>
    <td>Large</td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="xlarge"/></td>
    <td>X-Large</td>
  </tr>
    <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="xxlarge"/></td>
    <td>XX-Large</td>
  </tr>

  <tr>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
  </tr>
      <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="red"/></td>
    <td>Red</td>
  </tr>
   <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="black"/></td>
    <td>Black</td>
  </tr>
     <tr>
    <td>&nbsp;</td>
    <td><input type="checkbox" name="keyword[]"  value="white"/></td>
    <td>White</td>
  </tr>
</table>

<input type="submit" value="submit" />
</form>

my MYSQL database looks like this:

products table:

id  product_name  price  
5     shirt        20
6     shoe         70

I have an attributes table where i store the colors and size of the products.

this table looks like this:

idc  id  product_name  colors   sizes 
1     5     shirt        red     
2     5     shirt        back
3     5     shirt        white
4     5     shirt                small
5     5     shirt                medium
6     5     shirt                Large
7     6     shoe         brown
8     6     shoe         black
9     6     shoe                   5 
10    6     shoe                   6
11    6     shoe                   7

in my search.php page, I have this code :

include "config/connect.php"; 
$searchList = "";


foreach($_POST['keyword'] as $c){
$sql ="SELECT *
FROM `yt`
INNER JOIN `ATTRIBUTES` ON yt.id= ATTRIBUTES.id
WHERE (
     ATTRIBUTES.size LIKE '%".$c."%' OR
     ATTRIBUTES.color LIKE '%".$c."%'
      )";
$query = mysqli_query($db_conx, $sql);
}

//var_dump($query);

$productCount = mysqli_num_rows($query);
$i=0; // count the output amount
if ($productCount > 0) {
    while($row = mysqli_fetch_array($query, MYSQLI_ASSOC)){
             $id = $row["id"];
             $product_name = $row["product_name"];
             $searchList .= ''.$product_name.'';
    }
}

the code above doesn't return anything and I'm keep getting this error:

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given

which means there is something gone wrong after my sql query!

so I did var_dump($query); and that gave me bool(fales).

not much of a help as that means the same thing as the first error!

so could someone please help me out with this issue?

any advise and help would be appreciated.