在php中登录后，模态没有显示出来

Question

My modal is not showing up after I add header(location: homepage.php) but modal is working fine if I remove the header. How can I possibly do it? I've also tried using echo for alert and same thing happens, so I don't know what is wrong with my code. I hope someone can help me thank you!. Here is my code

login.php

if(isset($_POST['submit']))
{
    $email = $_POST['email'];
    $password = $_POST['password'];

    $object = new Login();
    $object->getCredentials($email, $password);
}
?>

<form  action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="POST">
<div>
 <label><b>EMAIL ADDRESS</b></label>
 <input type="email" name="email" placeholder="Email"></input>
</div>

<div>
 <label><b>PASSWORD</b></label>
 <input type="password" name="password" placeholder="Password"></input>
</div>
<button type="submit" id="submit" name="submit">Login</button>
</form>

processlogin.php

<?php
include_once 'db.php';
class Login
{
  public function getCredentials($email, $password)
  {     
    $email = $email;
    $password = $password;

    $object = new Db();
    $stmt = $object->connect()->prepare("SELECT * FROM user WHERE email=?");
    $stmt->execute([$email]);
    $stmtFetch = $stmt->fetch();

    if($stmt->rowCount()==1 && $stmtFetch['email'] == $email && $stmtFetch['password'] == $password)
    {
      echo "<script>$('#loginsuccess').modal('show')</script>";
      header("location: homepage.php");
    }
  }
}

Answer 1

I found something that might interest you here:
Interview Question: Can we have an echo before header?

The problem is that we cannot send the header after we start sending the output, and if you send the header before the echo, the echo will not be executed.

Try this:
Solution 1: (from the link above).

ob_start();
echo "<script>$('#loginsuccess').modal('show')</script>";
header("location: homepage.php");
ob_end_flush();

Solution 2: Use a javascript redirection instead of header function.

echo "<script>
        $('#loginsuccess').modal('show');
        window.location.replace('http://fullpath-homepage.php');
      </script>";

Additional note:
You can use also:

window.location.href="http://example.com";
window.location.assign("http://example.com");

The replace method navigates to the URL without adding a new record to the history.