Why mysql delete query is not working with string variable?

I am android developer and I am weak in PHP.

This one is working:

$sql = "DELETE FROM user WHERE id    =       '$user->id'    ";

This one works too:

$sql = "DELETE FROM user WHERE email = '" . '[email protected]' . "'";

But this query not working:

$sql = "DELETE FROM user WHERE email = '" .  "'$user->email'"  . "'";

This is my phpMyAdmin table:

and this is my del.php api that works fine with first two query:

<?php
 include_once('../common/include.php');
 include_once('../common/encipher.php');
 $user = json_decode(file_get_contents("php://input"));
 $conn=getConnection();
 if($conn==null){
     sendResponse(500,$conn,'Server Connection Error !');
     exit;
 }
 $sql = "DELETE FROM user WHERE id = '$user->id'";
 $result = $conn->query($sql);
 if ($conn->affected_rows > 0) {
 sendResponse(200, [], 'User deleted .');
 }
 else 
 {
  sendResponse(404, [], 'Error on delete !');
 }
 $conn->close();