Test if string could be boolean PHP

I’m getting a string from a $_GET and I want to test if it could be a boolean, before I use it for a part of a mysql query. Is there a better way of doing it than:

function checkBool($string){
    $string = strtolower($string);
    if ($string == "true" || $string == "false" || 
        $string == "1" || $string == "0"){
        return true;
    }
    else {
        return false;
    }
}

if (checkBool($_GET['male'])){
    $result = mysql_query(
        "SELECT * FROM my_table " .
        "WHERE male='".$_GET['male']."'") or die(mysql_error());
}

Answer

There’s, by the way, a cleaner way of writing it:

function checkBool($string){
    $string = strtolower($string);
    return (in_array($string, array("true", "false", "1", "0", "yes", "no"), true));
}

But yes. The one you wrote down is the only way.