Clean up registration and login names etc before inputting in mysql

Hi,
I am trying to clean up my html form inputs before they hit mysql.
Is this possible as I've shown below? (It's not currently working?)
If it will work have I missed anything?
Thanks

$username=  sanitise($_POST['username']);

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function sanitise($string) {
  $string = trim($string);
  $string = htmlentities($string);
  $string = strip_tags($string);
  $string = mysqli_real_escape_string($string);
  
  return $string;
}

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