php empty multidimensional array doesnt work! error: Undefined offset 0

Question

I have table in html code and I want put the table data in multdemensional array. and I have this error:

Undefined offset 0 ...

HTML code:

<td><input type="text" name="col[][]"  /></td>
<td><input type="text" name="col[][]"  /></td>
<td><input type="text" name="col[][]"  /></td>

PHP code:

 for ($k=0; $k<=2; $k++){ 

        for ($l=0; $l<=6; $l++){ //column
            echo $col[$k][$l];
        }
    }