str_ireplace() with keeping of case

How can I use str_ireplace (or something similar) to replace some text for formatting and then return it with the same caps?

Example:

$original="The quick red fox jumps over the lazy brown dog.";
$find="thE";

print str_ireplace($find,'<b>'.$find.'</b>',$original);

That will output: thE quick red fox jumps over the lazy brown dog.

I want it to keep the original case and only apply the formatting, in this example, bold text.

Thank you.