How to fix unusual problem in php, mysql? [duplicate]

I use a simple code for login to my website. After entering the username and password, I get this error:

ERROR:

408 Request Timeout in PHP MYSQL
Server timeout waiting for the HTTP request from the client.

Apache Server at otagh… Port 80

Note:
this error occurs only for one username&password
Other users login the site without problems

the output of database is correct for this username and password, and an error occurs only for this query in PHP.

the PHP script is executed correctly for all users, but a timeout error occurs only for the same

this is my php code:
$user = 'sys_user';
$pass = '1234';
if ($user && $pass) {     
    $sql = "SELECT * FROM users where user='$user'&& pass='$pass'";     
    $result = $mysqli->query($sql);     
    if ($result->num_rows > 0) {