is_alpha example

$tok = $this->scanner->next();

            if (false === $tok) {
                // end of string                 $this->parseError('Illegal tag opening');
            } elseif ('!' === $tok) {
                $this->markupDeclaration();
            } elseif ('/' === $tok) {
                $this->endTag();
            } elseif ('?' === $tok) {
                $this->processingInstruction();
            } elseif ($this->is_alpha($tok)) {
                $this->tagName();
            } else {
                $this->parseError('Illegal tag opening');
                // TODO is this necessary ?                 $this->characterData();
            }

            $tok = $this->scanner->current();
        }

        if (false === $tok) {