สอบถามการ echo แสดงว่าตัวแปรนั้นไม่มีค่าไหม สามารถดูได้ยังไงว่าตัวแปรนั้นมีค่หรือไหม

<?
include ("connect1.php");
$sql = "select a.*,b.* from borrowing_return equipment a,user b where a.user_id=b.user_id ";
<SPAN style="BACKGROUND-COLOR: red">echo $sql;</SPAN>
$sql .= " AND status_check != '2' AND status_check != '5' ";
$sql .= " ORDER BY a.borrow_id DESC";
$query = mysql_query($sql);
while ($arr = mysql_fetch_array($query))
{
?>

$sql = "select `borrowing_return equipment`.*,`user`.* from `borrowing_return equipment`,`user` where `borrowing_return equipment`.user_id=`user`.user_id ";
$query = mysql_query($sql);

$query = mysql_query(select `borrowing_return equipment`.*,`user`.* from `borrowing_return equipment`,`user` where `borrowing_return equipment`.user_id=`user`.user_id);