а на этой строке:
imagettftext($this->image, $this->textFontSize, $angle, $x, $y, $color, $_SERVER["DOCUMENT_ROOT"].$this->ttfFilesPath."/".$ttfFile, substr($this->code, $i, 1));
imagettftext($this->image, $this->textFontSize, $angle, $x, $y, $color, $_SERVER["DOCUMENT_ROOT"].$this->ttfFilesPath."/".$ttfFile, substr($this->code, $i, 1));