mysql - Set local variable for current row in SELECT -

i'm trying use variable row being selected 1 table in select function targetted towards second table. tried doing this:

while ($row = mysqli_fetch_assoc($result)) {     $sender = $row['sender_id'];      $sql = "select * users user_id='$sender'";     $result = mysqli_query($conn, $sql);      while ($row2 = mysqli_fetch_assoc($result)) {         echo $row2['user_uid'];     } } 

i understand not how set variable each row, have no clue how should done otherwise. @ moment displays username of sender of first notification in inbox database, not rest.

if knows how set variable per row, please let me know. thank in advance.