PHP - 重定向后执行函数

Question

I'm trying to create a login on login.php. When I submit, I redirect to the file validaton.php.

Now, I check if the username and the password is existing in my database with a simple function like this (there's more but that's what my question is about):

validation.php

$result =  $conn->query('SELECT * FROM db WHERE username  = '.$username.' AND password = '.$password);

if ($result->num_rows == 1){
  //Continue
  header("Location: welcome.php");
} else {
  // Go back to Login
  header("Location: login.php");
}

Let's say the user doesn't exist and the else-statement will be executed.

Is there a way to execute a function in login.php directly after I've been redirected from validation.php?

For example, I'd like to alert "This user doesn't exist!" on login.php after the redirect.

login.php

if(redirect happened){
  alert("This user doesn't exist!");
}

It's difficult to explain my question in English but I tried my best and any help would be highly appreciated! Thanks.

Answer 1

Yes, its possible. You need to append a parameter in your header().

Here is how you can do this:

$result =  $conn->query('SELECT * FROM db WHERE username  = '.$username.' AND password = '.$password);

if ($result->num_rows == 1){
  //Continue
  header("Location: welcome.php");
} else {
  // Go back to Login
  header("Location: login.php?redirect=1");
}

Now, you can check:

if($_GET['redirect'] == 1) {
 // redirect happened. execute some code or JS here.
}