Testing if a variable is a float string

Question

I know which type of variable $value should be. Should is_numeric() be used to test whether it is a float string?

private function sanitize($value, $type) {
    switch($type) {
        case 'boolean':
        if(!is_bool($value)) $value=filter_var($value, FILTER_VALIDATE_BOOLEAN);
        break;
        case 'integer':
        if(ctype_digit($value)) $value=(int)$value;
        break;
        case 'float':
        if(is_numeric($value) && !is_float($value)) $value=(float)$value;
        break;
        //case 'string':case 'object':case 'array':    //Not sanitized
    }
    return $value;
}