gistfile1.txt

$content = '\tThis variable is not set by me.\nCannot do anything about it.\n';

$replaced = preg_replace_callback(
                '/\\\\(\\\\|n|r|t|v|f|"|[0-7]{1,3}|\x[0-9A-Fa-f]{1,2})/',
                'replacer',
                $content);

var_dump($replaced);

function replacer($match) {
    $map = array(
        '\\\\' => "\\",
        '\\n' => "\n",
        '\\r' => "\r",
        '\\t' => "\t",
        '\\v' => "\v",
        // etc for \f \$ \"
    );

    $match = $match[0]; // So that $match is a scalar, the full matched pattern

    if (!empty($map[$match])) {
        return $map[$match];
    }

    // Otherwise it's octal or hex notation
    if ($match[1] == 'x') {
        return chr(hexdec(substr($match, 2)));
    }
    else {
        return chr(octdec(substr($match, 1)));
    }
}