のコメントが理解できない場合Juhana、これが彼の意味です。

echo "<td>";
echo "<select size=\"1\" name=\"pc\" id=\"pc\">"; 
echo "<option value=\"0\">Product Category</options>"; 
echo "<option value=\"77\">Hair</options>"; 
echo "<option value=\"88\">Skin</options>"; 
echo "<option value=\"99\">Nail</options>"; 
echo "<option value=\"66\">Eyes</options>"; 
echo "</select></td>";

ただし、JS / jQuery ソリューションを介してこれを行いたい場合。

jQuery

$(function() {
    $('#pc').change(function() {
        var cat_id = $('option:selected', this).data(category_id);
        $('#cat_id').val(cat_id);
    });
});

PHP

    echo "<td>";
    echo "<select size=\"1\" name=\"pc\" id=\"pc\">"; 
    echo "<option value=\"0\">Product Category</options>"; 
    echo "<option data-category_id=\"77\" value=\"Hair\">Hair</options>"; 
    echo "<option data-category_id=\"88\" value=\"Skin\">Skin</options>"; 
    echo "<option data-category_id=\"99\" value=\"Nail\">Nail</options>"; 
    echo "<option data-category_id=\"66\" value=\"Eyes\">Eyes</options>"; 
    echo "</select></td>";
    echo "<input type=\"hidden\" id=\"cat_id\" name=\"category_id\" />";

これらの ID に価値を置かないのはなぜですか?

echo "<tr><td>Product Category* </td>";

echo "<td>";
echo "<select size=\"1\" name=\"pc\" id=\"pc\">"; 
echo "<option value=\"0\">Product Category</options>"; 
echo "<option value=\"77\">Hair</options>"; 
echo "<option value=\"88\">Skin</options>"; 
echo "<option value=\"99\">Nail</options>"; 
echo "<option value=\"66\">Eyes</options>"; 
echo "</select></td>";