Internal Linking Plugin For Modx

$output = &$modx->documentOutput;
$output = replace_first(" madonna ", '<a href="madonna.html"> madonna </a>', $output);
return $output;

function replace_first($search, $replace, $data) {
    $res = strpos($data, $search);
    if($res === false) {
        return $data;
    } else {
        // There is data to be replaced
        $left_seg = substr($data, 0, strpos($data, $search));
        $right_seg = substr($data, (strpos($data, $search) + strlen($search)));
        return $left_seg . $replace . $right_seg;
    }
}

<a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"><a href="http://www.example.com/madonna"> madonna </a></a></a></a></a></a></a></a></a>

if (!function_exists('str_replace_once')) {
    function str_replace_once($needle , $replace , $haystack){
        // Looks for the first occurence of $needle in $haystack
        // and replaces it with $replace.
        $pos = strpos($haystack, $needle);
        if ($pos === false) {
            // Nothing found
            return $haystack;
        }
        return substr_replace($haystack, $replace, $pos, strlen($needle));
    }
} 
$output = &$modx->documentOutput;
$output = str_replace_once(" maddona ", ' <a href="maddona.html">maddona</a> ', $output); //notice str_replace_once
$output = str_replace_once(" RedBull ", ' <a href="redbull.html">RedBull</a> ', $output); //instead of str_replace
return $output;

if (!function_exists('myFunction')) {
function myFunction()...
}