How do you make strings "XML safe"?

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I am responding to an AJAX call by sending it an XML document through PHP echos. In order to form this XML document, I loop through the records of a database. The problem is that the database includes records that have '<' symbols in them. So naturally, the browser throws an error at that particular spot. How can this be fixed?

cakephp xml php
7 Answers

I prefer the way Golang does quote escaping for XML (and a few extras like newline escaping, and escaping some other characters), so I have ported its XML escape function to PHP below

function isInCharacterRange(int $r): bool {
    return $r == 0x09 ||
            $r == 0x0A ||
            $r == 0x0D ||
            $r >= 0x20 && $r <= 0xDF77 ||
            $r >= 0xE000 && $r <= 0xFFFD ||
            $r >= 0x10000 && $r <= 0x10FFFF;
}

function xml(string $s, bool $escapeNewline = true): string {
    $w = '';

    $Last = 0;
    $l = strlen($s);
    $i = 0;

    while ($i < $l) {
        $r = mb_substr(substr($s, $i), 0, 1);
        $Width = strlen($r);
        $i += $Width;
        switch ($r) {
            case '"':
                $esc = '&#34;';
                break;
            case "'":
                $esc = '&#39;';
                break;
            case '&':
                $esc = '&amp;';
                break;
            case '<':
                $esc = '&lt;';
                break;
            case '>':
                $esc = '&gt;';
                break;
            case "\t":
                $esc = '&#x9;';
                break;
            case "\n":
                if (!$escapeNewline) {
                    continue 2;
                }
                $esc = '&#xA;';
                break;
            case "\r":
                $esc = '&#xD;';
                break;
            default:
                if (!isInCharacterRange(mb_ord($r)) || (mb_ord($r) === 0xFFFD && $Width === 1)) {
                    $esc = "\u{FFFD}";
                    break;
                }

                continue 2;
        }
        $w .= substr($s, $Last, $i - $Last - $Width) . $esc;
        $Last = $i;
    }
    $w .= substr($s, $Last);
    return $w;
}

Note you'll need at least PHP7.2 because of the mb_ord usage, or you'll have to swap it out for another polyfill, but these functions are working great for us!

For anyone curious, here is the relevant Go source https://golang.org/src/encoding/xml/xml.go?s=44219:44263#L1887

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