strrchr with strtr or str_replace in PHP

I have searched a lot of sites and SO answers for replacing characters in strings, but I didn't find a solution.

I need to replace only the last used character 'é' of a string. It should work for every string, also if the string only contains once character 'é'.

code

echo strtr (strrchr ( 'accélérer','é' ), array ('é' => 'è'));   //  output èrer
echo str_replace("é","è",strrchr ( 'accélérer','é' ));           // output èrer

desired results

accélérer -> accélèrer

sécher-> sècher

Have created custom function. Might be useful:

<?php
$str = 'accélérer';
$output = replaceMultiByte($str, 'é', 'è');
echo "OUTPUT=".$output; // accélèrer
echo '<br/><br/>';

$str = 'sécher';
$output = replaceMultiByte($str, 'é', 'è');
echo "OUTPUT=".$output; // sècher

function replaceMultiByte($str, $replace, $replaceWith)
{
    $exp = explode($replace, $str);

    $i = 1;
    $cnt = count($exp);
    $format_str = '';
    foreach($exp as $v)
    {
        if($i == 1)
        {
            $format_str = $v;
        }
        else if($i == $cnt)
        {
            $format_str .= $replaceWith . $v;
        }
        else
        {
            $format_str .= $replace . $v;
        }
        $i++;
    }
    return $format_str;
}
?>