Leafo\ScssPhp\Compiler::libFunctionExists PHP Method

    protected function libFunctionExists($args)
    {
        $string = $this->coerceString($args[0]);
        $name = $this->compileStringContent($string);
        // user defined functions
        if ($this->has(static::$namespaces['function'] . $name)) {
            return true;
        }
        $name = $this->normalizeName($name);
        if (isset($this->userFunctions[$name])) {
            return true;
        }
        // built-in functions
        $f = $this->getBuiltinFunction($name);
        return $this->toBool(is_callable($f));
    }