Forget all of the mb_substr_replace() implementations mentioned in this page, they're all buggy.

Here is a version that mimics the behavior of substr_replace() exactly:

<?php

if (function_exists('mb_substr_replace') === false)
{
    function mb_substr_replace($string, $replacement, $start, $length = null, $encoding = null)
    {
        if (extension_loaded('mbstring') === true)
        {
            $string_length = (is_null($encoding) === true) ? mb_strlen($string) : mb_strlen($string, $encoding);
            
            if ($start < 0)
            {
                $start = max(0, $string_length + $start);
            }
            
            else if ($start > $string_length)
            {
                $start = $string_length;
            }
            
            if ($length < 0)
            {
                $length = max(0, $string_length - $start + $length);
            }
            
            else if ((is_null($length) === true) || ($length > $string_length))
            {
                $length = $string_length;
            }
            
            if (($start + $length) > $string_length)
            {
                $length = $string_length - $start;
            }
            
            if (is_null($encoding) === true)
            {
                return mb_substr($string, 0, $start) . $replacement . mb_substr($string, $start + $length, $string_length - $start - $length);
            }
            
            return mb_substr($string, 0, $start, $encoding) . $replacement . mb_substr($string, $start + $length, $string_length - $start - $length, $encoding);
        }
        
        return (is_null($length) === true) ? substr_replace($string, $replacement, $start) : substr_replace($string, $replacement, $start, $length);
    }
}

?>