Where am I going wrong with the function below?

[christyPaddy]

function sanitize_string(string $var)
{
    $var = strip_tags($var);
    $var = htmlentities($var);
    $var = stripslashes($var);
    $var = trim($var);
    return mysqli_real_escape_string($var);
}

[Dormilich]

Where am I going wrong with the function below?

nowhere.

which sanitising function to use strongly depends on the context change, e.g.
- when using prepared statements, escaping for SQL is unnecessary
- htmlspecialchar s() is only for printing into HTML/XML and nothing else.
- strip_tags() is only against XSS
- a decently recent PHP installation doesn’t add slashes and stripping slashes off a windows directory name is not the best of ideas
- trimming a string is a bad idea if you need to retain whitespace

essentially, you always have to decide which sanitisation you need depending on the circumstances.