php mysql query from a field in a table

Question

I want the query from a field in a table. It works oke. But when i put a variable in it, it won't work anymore.

example:

$file1 = "usr.php";
include("conn.php");
$usnm = 'john';
$sql1 = "select id, name, place from  users where name = $usnm ";


                $sql = "SELECT  * FROM vg_db where file = '$file1'"; 
                    $result = $conn->query($sql);
    if ($result->num_rows > 0) {while($row = $result->fetch_assoc()) { 
// in field db_grid => "select id, name, place from  users where name = $usnm ";    
        $sql2= $row["db_grid"];
    }}

// $sql1 works      $usnm in $sql1 => john
            
                    $result = $conn->query($sql1);
    if ($result->num_rows > 0) {while($row = $result->fetch_assoc()) {  
        $place= $row["place"];
    }}


// $sql2 won't work    $usnm in $sql2 => $usnm
            
                    $result = $conn->query($sql2);
    if ($result->num_rows > 0) {while($row = $result->fetch_assoc()) {  
        $place= $row["place"];
    }}