Converts plain text to HTML

TextHtmlConverter.php

<?php

/**
 * Converts a text message to HTML.
 *
 * The following transformations are applied:
 * - Special characters are encoded to HTML entities,
 * - Lines of text separated by an emtpy line are converted to paragraphs,
 * - Newline characters are converted to `<br>`,
 * - Links and e-mail addresses are converted to HTML links,
 * - Tab-separated data is converted to tables.
 */
class TextHtmlConverter
{
    /**
     * @var bool
     */
    private $isXhtml;

    /**
     * @var bool
     */
    private $prettyPrint;

    /**
     * @var int
     */
    private $indentLevel = 0;

    /**
     * @var string
     */
    private $html;

    /**
     * Class constructor.
     *
     * @param bool $isXhtml
     * @param bool $prettyPrint
     */
    public function __construct(bool $isXhtml = true, bool $prettyPrint = false)
    {
        $this->isXhtml     = $isXhtml;
        $this->prettyPrint = $prettyPrint;
    }

    /**
     * @param string $text
     *
     * @return string
     */
    public function convert(string $text) : string
    {
        $this->html = '';

        $lines = $this->splitLines($text);

        $groups = [];
        $group  = [];

        foreach ($lines as $line) {
            if ($line != '') {
                $group[] = $line;
            } else {
                if ($group) {
                    $groups[] = $group;
                    $group = [];
                }
            }
        }

        if ($group) {
            $groups[] = $group;
        }

        foreach ($groups as $lines) {
            $isTable = false;

            foreach ($lines as $line) {
                if (strpos($line, "\t") !== false) {
                    $isTable = true;
                    break;
                }
            }

            if ($isTable) {
                $rows = [];

                foreach ($lines as $line) {
                    $rows[] = explode("\t", $line);
                }

                $this->convertTable($rows);
            }
            else {
                $this->convertParagraph($lines);
            }
        }

        return $this->html;
    }

    /**
     * Splits a string into an array of lines.
     *
     * @param string $text
     *
     * @return array
     */
    private function splitLines(string $text) : array
    {
        $text = str_replace("\r\n", "\n", $text);
        $text = str_replace("\r", "\n", $text);

        return explode("\n", $text);
    }

    /**
     * @param string $html
     * @param int    $level
     *
     * @return void
     */
    private function append(string $html, int $level = 0) : void
    {
        if ($this->prettyPrint) {
            if ($level < 0) {
                $this->indentLevel--;
            }
            $this->html .= str_repeat(' ', 4 * $this->indentLevel) . $html . PHP_EOL;
            if ($level > 0) {
                $this->indentLevel++;
            }
        } else {
            $this->html .= $html;
        }
    }

    /**
     * Converts an array of table data into an HTML table.
     *
     * @param array $rows
     *
     * @return void
     */
    private function convertTable(array $rows) : void
    {
        $this->append('<table>', +1);

        foreach ($rows as $row) {
            $this->append('<tr>', +1);

            foreach ($row as $column) {
                $this->append('<td>' . $this->encodeHtmlWithLinks($column) . '</td>');
            }

            $this->append('</tr>', -1);
        }

        $this->append('</table>', -1);
    }

    /**
     * @param array $lines
     *
     * @return void
     */
    private function convertParagraph(array $lines) : void
    {
        $br = $this->isXhtml ? '<br />' : '<br>';
        $last = count($lines) - 1;

        $this->append('<p>', +1);

        foreach ($lines as $index => $line) {
            $line = $this->encodeHtmlWithLinks($line);

            if ($index != $last) {
                $line .= $br;
            }

            $this->append($line);
        }

        $this->append('</p>', -1);
    }

    /**
     * Encodes a plain text string in HTML.
     *
     * @param string $text The text to encode.
     *
     * @return string The encoded text.
     */
    private function encodeHtml(string $text) : string
    {
        return htmlspecialchars($text, ENT_COMPAT | ENT_XHTML, 'UTF-8');
    }

    /**
     * Creates an HTML version of a plain text message.
     *
     * * Encodes special characters with html entities.
     * * Adds anchors around emails and hyperlinks.
     *
     * @param string $text The plain text message.
     *
     * @return string The HTML message.
     */
    private function encodeHtmlWithLinks(string $text) : string
    {
        $html = $this->encodeHtml($text);

        $replacements = [
            '*@*.*'   => '<a href="mailto:$0">$0</a>',
            '*://*.*' => '<a href="$0" target="_blank">$0</a>',
        ];

        foreach ($replacements as $pattern => $replacement) {
            $html = preg_replace($this->buildPattern($pattern), $replacement, $html);
        }

        return $html;
    }

    /**
     * @param string $string
     *
     * @return string
     */
    private function buildPattern(string $string) : string
    {
        // Non-blank ASCII chars excluding <> HTML tag characters.
        // This exclusion is required in case the link touches an HTML tag.
        $chars = '[^\<\>\x00-\x20\x7f-\xff]+';

        $string = preg_quote($string, '/');
        $string = str_replace('\\*', $chars, $string);

        return '/' . $string . '/';
    }
}