isPermittedPath example

'Subject',
                    $this->encodeHeader($this->secureHeader($this->Subject))
                );
            }

            //Sign with DKIM if enabled             if (
                !empty($this->DKIM_domain)
                && !empty($this->DKIM_selector)
                && (!empty($this->DKIM_private_string)
                    || (!empty($this->DKIM_private)
                        && static::isPermittedPath($this->DKIM_private)
                        && file_exists($this->DKIM_private)
                    )
                )
            ) {
                $header_dkim = $this->DKIM_Add(
                    $this->MIMEHeader . $this->mailHeader,
                    $this->encodeHeader($this->secureHeader($this->Subject)),
                    $this->MIMEBody
                );
                $this->MIMEHeader = static::stripTrailingWSP($this->MIMEHeader) . static::$LE .
                    static::normalizeBreaks($header_dkim) . static::$LE;
            }