<tr>
<td>data1</td>
<td>data2</td>
<td>data3</td>
<td>data4</td>
<td>data5</td>
<td>data6</td>
<td>data7</td>
<td>data8</td>
</tr>

$sql = 'SELECT `something` FROM `table` WHERE ';

$i = 0;
foreach($_POST as $key => $val) {
    if (!empty($val)) {
        if ($i != 0)
           $sql .= ' AND ';

        $sql .= '`'.$key'` = "'.$val.'"';
        $i++;
    }
}

$test = ['test1' => 111, 'test2' => '', 'test3' => '', 'test4' => 'sdfsdf'];

$postVars = array_filter($test, function($el){
    return !empty($el);
});